A piece of wood is solid. A rock is solid. You cannot change its shape unless you break it.

Water is not solid. Water will change shapes when you pour it from one container to another. Water will always go to the bottom of the container.

Air is a gas. Gases will change shapes like water. Gases will expand (get bigger) to fill the container. You can push gases into a smaller container, too, like a balloon.

Water and air are called "fluids". Fluids will take the shape of the container.

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Last modified: Sat Jun 20 21:18:09 PDT 1998