Marshmallows!

Here are D'artagnan and Harriet the marshmallows, happily in love (and unaware of their impending doom).

After 1 minute in the microwave, they look quite a bit different. So what happens?

Marshmallows are colloids (permanently suspended homogeneous mixtures) of sugar and air. When the marshmallow is heated, the air particles get excited and move faster and more, so the air expands and so does the marshmallow.

When the microwave cuts off, the air cools down dramatically, the gas particles contract, and the marshmallow shrinks.

Now you know... and knowing is half the battle.