Great Question!
Everything around us is made up of matter. (Matter is just a fancy word for stuff.) If you cut matter up as small as you possibly can - much smaller than you can see with your eyes, or even a microscope - then you get what scientists like to call atoms. There's lots of different kinds of atoms, like oxygen, nitrogen, and hydrogen atoms. You may have learned about some of these in school. When you put several atoms together, they "bond" (or stick) together and you get what's called molecules.
A water molecule is what you get when you put together two hydrogen atoms and one oxygen atom. The shape of the water molecule has the oxygen atom in the middle and the two hydrogen atoms stuck to it on the sides, and it sort of makes a triangle. It looks a little bit like this, where the O is the Oxygen atom, the H's are the Hydrogen atoms, and the lines are the bonds between them:
O
/ \
H H
Other molecules (for example, different types of chemicals) have different shapes.
When a liquid (like water) is frozen, all of the molecules start sticking to each other and holding on very tightly. Because different types of molecules have different shapes, they hold on to each other in different places. Most of the time, when they start holding on to each other, they get closer together. When the molecules get closer together, they take up less space, so the frozen solid ends up being smaller than the unfrozen liquid.
Water, however, is a bit weird. When the water molecules start holding on to each other really tightly, they make a pattern that actually takes up /more/ space than they did when they weren't stuck together. (This pattern is what you see if you look at ice crystals.) So, when water freezes, the molecules take up more space, and the ice ends up being even /bigger/ than the water was.
If you were to put that water in a closed container in the freezer, then it would still get bigger. What happens to the container depends on what sort of a container it is. For example, if the container were made of thin plastic, it would probably stretch a bit as the water freezes. But if you were to put it in a very full, tightly sealed glass container, then the frozen water would be pushing so hard that the glass might break. This is why if you put a glass bottle of juice in the freezer, you're supposed to take the lid off until it's frozen all the way.
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