Measuring

Watching for a tiny dip

How Do We Find Planets Around Other Stars?

Most exoplanets are found by the tiny repeating dip in starlight as the planet passes in front.

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An island of stars

What Is a Galaxy?

A galaxy is an enormous gravity-bound island of stars, separated from other galaxies by near-total emptiness.

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Yes, lots

Is There Gravity in Space?

Gravity fills all of space; it just weakens with distance and never quite stops.

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A distance, not a time

What Is a Light Year?

A light year is a distance — how far light travels in twelve months.

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Further than you think

How Far Away Is the Moon?

The Moon is 384,000 km away — far enough to fit every other planet in between.

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Too far away to fall behind

Why Does the Moon Follow the Car?

The Moon appears to follow you because it is far too distant for your travel to shift its direction at all.

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A definition, not a demotion

Why Is Pluto Not a Planet Any More?

Pluto lost its planet status because astronomers found many similar objects sharing its region of space.

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Bigger than your brain wants

How Big Is the Sun?

The Sun is so big that a million Earths would fit inside it.

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Pressure from below

Why Does Water Push Up?

Buoyancy exists because water pressure is greater at the bottom of an object than the top.

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Heavy for its size

What Is Density?

Density is how much mass is squeezed into a given amount of space.

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Waves bunched and stretched

Why Does a Siren Change Pitch as It Passes?

A passing siren drops in pitch because its waves arrive bunched while approaching and stretched once leaving.

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Water pushes back

Why Do Things Float?

Things float when the water they push aside weighs more than they do.

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The pull you never notice

What Is Gravity?

Gravity is the pull every object with mass has on every other object.

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Everything is shaking

What Is Heat?

Heat is the movement of atoms — the faster they jiggle, the hotter it is.

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Trading distance for force

How Do Levers Work?

A lever lets a small push move a big load by trading distance for force.

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How big the push is

What Makes a Sound Loud?

Loudness comes from how big the air's squeeze is, not how fast it repeats.

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Mass times speed

Why Are Heavy Things Hard to Stop?

Stopping something depends on both its mass and its speed, and spreading the stop over time reduces the force.

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Counting wiggles

What Makes a Sound High or Low?

Pitch is how fast something vibrates — fast means high, slow means low.

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Force, spread out

What Is Pressure?

Pressure is force divided by area — a small area concentrates the same push.

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Sharing the load

How Do Pulleys Help?

Pulleys split a heavy load across several rope sections so each pull is lighter.

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A skin made of pulling

Why Does a Paperclip Float on Water?

A paperclip floats because water molecules cling to each other, giving the surface a skin that resists being broken.

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Pieces too small to see

What Is Everything Made Of?

Everything is built from atoms, and there are only about a hundred different kinds to build with.

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How hard the atoms hold on

Why Do You Need a Furnace to Melt Iron?

Metals melt at wildly different temperatures because their atoms grip each other with wildly different strength.

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Putting together

What Does Adding Do?

Adding puts two groups together into one bigger total.

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Turn, not length

What Is an Angle Actually Measuring?

An angle measures amount of turn between two directions, so longer arms do not make a bigger angle.

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Counting squares, quickly

Why Do We Multiply to Find Area?

Area is the number of squares covering a shape, and multiplying rows by columns counts them fast.

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Levelling everyone off

What Does an Average Actually Tell You?

An average is what everyone would get if the total were shared equally, which can hide a lot.

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A thousand times, not a bit

How Much Bigger Is a Billion Than a Million?

A billion is a thousand times a million, and turning both into seconds shows how vast that gap is.

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Objects have no memory

Why Does a Coin Not Owe You Heads?

Chance has no memory: past results cannot influence a fair coin or die on its next throw.

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Counting up, not taking away

How Do You Work Out Your Change?

Change is easiest worked out by counting up from the price to the note you handed over.

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Counting something reliable

How Do Clocks Tell Time?

A clock counts something that repeats at a perfectly steady rate.

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Two numbers, one place

How Do You Describe Exactly One Square?

Coordinates pin down a place with two numbers: how far across, then how far up.

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Count a little, multiply a lot

How Do You Count a Huge Crowd?

Crowds are counted by counting one small area and multiplying by the number of areas.

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Taking a slice off the top

How Do Shops Work Out a Sale Price?

A discount takes a percentage off the old price, so the saving depends on what the price was.

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Balance, not 'the answer'

What Does the Equals Sign Really Mean?

The equals sign means both sides balance, which is why 7 = 4 + 3 is perfectly correct.

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Close enough on purpose

What Is Estimating?

Estimating gives you a fast, roughly right answer instead of a slow exact one.

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Bigger bottom, smaller piece

Why Is a Third Bigger Than a Quarter?

A third beats a quarter because the bottom number counts cuts, and more cuts make each piece smaller.

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Seeing beats reading

Why Draw Numbers as a Picture?

Graphs work because eyes compare lengths and heights far faster than they read columns of numbers.

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Comparing

Which Number Is Bigger?

A bigger number is simply one you reach later when counting.

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Two equal shares

What Does Half Mean?

A half is one of two pieces that are exactly the same size.

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Never running out

What Is Infinity?

Infinity means there is no end — you can always add one more.

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Everything divided by the same number

How Does a Map Fit a Country on a Page?

A map shrinks every distance by the same factor, which is what keeps the shapes right.

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Everyone agreeing

Why Do We Measure With Units?

Measuring means counting how many agreed units something is.

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One giant number spoils it

Why Is the Average Sometimes a Lie?

An average is dragged by extreme values, while the middle value ignores them.

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Making any amount

Why Do Coins Have Different Values?

Coin values are chosen so you can make any amount with only a few coins.

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The line carries on

How Can a Number Be Less Than Nothing?

Negative numbers exist because the number line does not stop at zero; it carries straight on.

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Numbers as places

What Is a Number Line?

A number line turns numbers into positions, so arithmetic becomes movement.

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Out of a hundred, always

What Does Per Cent Actually Mean?

Per cent means out of a hundred, which lets you compare quantities of different sizes.

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The fence and the field

How Are Area and Perimeter Different?

Perimeter is the distance round the edge and area is the space inside, and one does not fix the other.

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Every circle's hidden number

What Is Pi and Why Does It Never End?

Pi is the number of diameters that fit around any circle, and it is the same for all of them.

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Position carries meaning

Why Does 25 Mean Twenty-Five?

In our number system, where a digit sits decides how much it is worth.

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One of four

What Is a Quarter?

A quarter is one of four exactly equal shares of a whole.

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You are usually in the slow one

Why Is the Other Queue Always Faster?

With several queues, the odds are against yours being the quickest — and one shared queue fixes it.

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Two out of a million is still nearly none

Why Does Buying Two Tickets Barely Help?

Doubling your tickets doubles a chance so small that twice it is still almost nothing.

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Position is not distance

Why Do Rankings Change When Nothing Does?

A ranking keeps only the order, discarding how far apart the things really are.

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Not everything scales

How Do You Double a Recipe?

Doubling a recipe doubles the ingredients but not the pan, the time, or the temperature.

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Close enough, on purpose

Why Do We Round Numbers?

Rounding deliberately gives up small accuracy to get a number that is easier to think with.

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Length halves, weight drops by eight

Why Is a Model Never Quite Right?

Scaling changes length, area and volume by different factors, so models never behave like the real thing.

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Sorted by rules

Why Do Shapes Have Names?

Shape names are definitions — a checklist of sides, angles and corners.

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A third direction

What Makes a Shape 3D?

A solid shape adds a third direction to a flat one, which is what gives it faces, edges and corners.

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One triangle, three questions

How Long Will the Journey Take?

Speed, distance and time form one relationship you can rearrange to answer any of the three.

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Sharing, with an awkward remainder

How Do You Split a Bill Fairly?

Splitting a bill is division until there is a remainder, and then it is a decision.

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Two matching halves

What Is Symmetry?

Symmetry means a shape matches itself when folded or turned.

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What is left

What Does Taking Away Do?

Subtraction finds what is left, or how far apart two numbers are.

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A scale built around water

Why Does Water Freeze at Zero?

Water freezes at zero because the Celsius scale was deliberately built around water's two fixed points.

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Sharing between three

What Is a Third?

A third is one of three exactly equal shares — and it never divides evenly in decimals.

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Noon cannot be everywhere at once

Why Is It a Different Day Somewhere Else?

Time zones exist because noon happens at different moments as the Earth turns.

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Ten per cent, then adjust

How Do People Work Out a Tip in Their Head?

A tip is worked out by finding ten per cent and adjusting up or down, not by exact sums.

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The number under the price

Which Packet Is Actually Cheaper?

Unit price divides cost by amount so packets of different sizes can be compared.

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Stacking layers of squares

How Much Does a Box Actually Hold?

Volume is the number of unit cubes that fit inside a space, found by stacking layers of area.

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Size and heaviness are separate

Why Is a Big Thing Not Always Heavy?

Big does not mean heavy, because weight depends on how tightly matter is packed, not on size.

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A number your eye reads first

Why Does Everything Cost £4.99?

Prices end in 99 because people read the first digit and round the rest away.

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A number for nothing

What Is Zero?

Zero is the number that counts nothing, and it makes big numbers possible.

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Weighing the sky

How Does a Barometer Work?

A barometer measures the weight of air overhead, which falls before storms and rises before fine weather.

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The temperature where air gives up

What Is the Dew Point?

The dew point is the temperature at which air can no longer hold its water as vapour.

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One-ninth showing

Why Do Icebergs Float So Low?

Ice is only slightly lighter than water, so about nine-tenths of an iceberg is submerged.

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Counting between flash and bang

How Far Away Is the Storm?

Count the seconds between lightning and thunder and divide by three for the distance in kilometres.

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Lines, triangles and half-circles

How Do You Read a Weather Map?

Weather map lines join places of equal pressure, and the fronts show where air masses collide.

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Named for where it starts

Where Does the Wind Come From?

Wind is named for the direction it comes from, which tells you what kind of air is arriving.

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Cartilage turning into bone

How Do You Grow Taller?

You grow taller at soft zones near the ends of your bones, mostly during sleep.

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A name that is probably wrong

Do Growing Pains Really Come From Growing?

Growing pains are real aches with a misleading name — bones grow far too slowly and too painlessly to be the cause.

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A saltiness detector

How Does Your Body Know You Need a Drink?

Thirst comes from a patch of brain measuring how salty your blood is, not from a dry mouth.

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A bag that reports on itself

Why Can't You Just Hold It Forever?

Your bladder measures stretch, not volume — which is why the urge comes in waves and why cold makes it worse.

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A grain that drinks

Why Does Rice Need Exactly the Right Water?

Rice grains swell by absorbing a set amount of water, so the measure matters more than the time.

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Thickness, tightness, length

Why Do Guitar Strings Sound Different?

A string's note depends on how thick, how tight and how long it is — and a guitar lets you change all three.

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Blocking light, close and far

How Do Shadow Puppets Work?

Shadow puppets work because blocked light spreads out, so nearer objects cast bigger shadows.

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The same note, a different mix

Why Does a Piano Not Sound Like a Violin?

Instruments differ because each adds a different mix of quieter extra notes on top of the one being played.

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A see-saw with a concrete end

Why Doesn't a Crane Tip Over?

A crane stays up because it is a see-saw: concrete on the short arm balances the load on the long one.

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Four clocks and some arithmetic

How Does a Phone Know Where It Is?

Nothing tracks your phone: it times signals from several satellites and works out the only place it could be.

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