Why does water boil at 100 degrees Celsius and freeze at 0?

Water boils at 100 degrees Celsius and freezes at 0 because it changes from a liquid to a gas or a solid when its molecules get enough energy or lose enough of it.

When you heat up water, like in a pot on the stove, the molecules inside it start moving faster. They bounce around more and farther apart. When they reach 100 degrees Celsius, they move so fast that they can escape from the liquid and become gas, which we see as steam.

When you cool down water, like in a freezer, the molecules slow down and start moving closer together. At 0 degrees Celsius, they get so close and still that they form a solid, which is ice.

It’s like when you're playing with marbles. When you shake them up fast, they fly apart, that's like boiling. But when you let them settle down slowly, they stack neatly, that's like freezing.