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Steam bursting out

Why Does Popcorn Pop?

Water sealed inside the kernel turns to steam, and the pressure finally bursts the shell.

1 A kernel withwater sealedinside2 Heat turns itto steam3 Pressure burststhe shell
A tiny steam explosion, every time.

  1. Step 1 of 5

    Each kernel has water sealed inside.

    A kernel About 14% water, sealedin by the shell

    Popcorn holds around 14 percent water inside a hard, unusually waterproof shell. That combination is what makes it pop.

  2. Step 2 of 5

    Heating turns that water into steam.

    As water Tiny volumeAs steam Over 1,000× the space

    Past 100°C the water wants to become vapour, which takes up over a thousand times more space. The shell will not let it.

  3. Step 3 of 5

    Pressure builds enormously.

    Inside the kernelabout 9×atmosphericpressureNormal air1 atmosphere

    Sealed in, the steam reaches around nine times atmospheric pressure and the inside heats to about 180°C before anything gives way.

  4. Step 4 of 5

    The shell fails all at once.

    1 The shellsplits2 Starch expandsinstantly3 It sets aswhite foam

    When it splits, pressure drops instantly. The hot starch inside expands violently and turns inside out into white foam.

  5. Step 5 of 5

    Other corn does not do this.

    Sweetcorn Porous shell — steamescapesPopcorn Sealed shell — pressurebuilds

    Sweetcorn's shell is porous, so steam leaks out gently and nothing builds up. Only popcorn's shell is sealed tightly enough.

    Kernels that fail to pop usually have a crack in the shell that let the moisture escape beforehand.

The short version

Popcorn pops because water sealed in a waterproof shell becomes high-pressure steam and bursts it open.

Try it yourself

Look at an unpopped kernel next to a popped one. The popped one is turned inside out — the shell is on the outside.

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