Why do potato slices go crunchy in the oven?

Have you ever bitten into a baked potato slice and heard it go crunch? Think of the warm, salty smell drifting from the oven and that crisp snap between your teeth. Before baking, that same slice was soft and floppy, and it bent like a wet leaf.

A grown-up slides the tray into the hot oven, and later out come slices that snap. So why do potato slices go crunchy in the oven? Let's follow one slice and find out.

A raw potato slice is full of water, hidden in little spaces all through it. That water keeps the slice soft and bendy, the same way water keeps a sponge squishy.

Inside the hot oven, the heat warms the water in the slice until it turns into steam. The steam floats out of the slice and away into the oven air. Bit by bit, the slice gets drier and lighter.

Because the slice is thin, the steam has only a short way to travel out. So the slice dries all the way through, from its edges to its middle. A thick chunk of potato keeps water deep inside and stays soft there.

Once the water has gone, something stays behind. Most of what is left is starch, which means the small grains of food that fill every potato and make it feel floury. What do you think happens to the starch once its water is gone?

The starch dries and sets into a hard, stiff layer, much like glue drying on paper. Now the slice is brittle, which means hard and easy to snap. It can no longer bend the way it did before.

When you bite down, the brittle starch cannot bend, so it cracks into lots of pieces at once. Each crack makes a small snap, and all those snaps together are the crunch you hear.

Starch, brittle. The oven's heat turns the water inside a potato slice into steam that floats away, and the dry starch left behind sets brittle, so it cracks with a crunch when you bite.

Next time you hear that crunch, you will know just what made it. Sleep warm and snug tonight.