Some vegtables are being cooked in a pot on the stove. The lid starts to jump up and down what occur this (Sie

Question

Some vegtables are being cooked in a pot on the stove. The lid starts to jump up and down what occur this (Sie

Answer 1

When water reaches 100 degress celsius (or more if it is tap water) it boils and becomes a gas - steam.

As more steam forms the pressure builds up in the pan. When the pressure gets high the steam forces its way out, lifting the lid up to do so. This keeps happening because only a small amount of steam escapes at a time.