A pond in a box
Why Do Animals Lay Eggs?
An egg is a private pond with a packed lunch, sealed inside a shell that still lets air through.
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Step 1 of 5
The yolk is the food.
The yellow is not the baby. It is the food store, and the chick grows on a small pale spot on its surface, slowly using the yolk up until there is nothing left on hatching day.
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Step 2 of 5
The white is the water.
A growing animal cannot dry out. The clear white is mostly water with protein in it, and it doubles as a shock absorber, holding the yolk suspended in the middle away from the shell.
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Step 3 of 5
The shell breathes.
A shell has thousands of microscopic holes. Air gets in and waste gas gets out through them, which is exactly why sealing an egg in something airtight kills what is inside.
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Step 4 of 5
The shape is astonishingly strong.
A whole fist can fail where a teaspoon succeeds Press an egg end to end in your fist and it is very hard to break — the curve spreads the push all around the shell. Tap the side with a spoon and it goes at once.
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Step 5 of 5
Not laying eggs costs the mother more.
Laying means the mother can put the young down and go and eat. Carrying them inside means she is never free of the weight, but nothing can steal them. Different animals took different deals.
Not only birds lay eggs. So do snakes, frogs, fish, insects, and two odd mammals — the platypus and the echidna.
The short version
An egg holds water, food and air for a growing animal, inside a shell strong along its curve and full of holes for breathing.
Try it yourself
Wrap your whole hand round an egg and squeeze evenly. It holds. Now you can see why a bird can sit on a nest full of them.