Hard, wet, and invisible
Which is strongest—a stone wall, running water, or blowing wind?
A stone wall can stop the wind. But running water can wear down a stone
wall. And moving air in the wind can dry up water. So, each of them is
strongest—in a different way.
The wall is hard, the water is wet, and the wind is invisible. But
stone, water, and air are all matter. They are all made up of molecules.
They are different forms of matter—they behave in different ways.
Stone is a solid [(sahl]{.smallcaps} ihd)—it has a shape of its own.
The molecules in solids are very close together. They pull hard on each
other—much as magnets do. This pull makes solids keep their shape.
Water is a liquid [(lihk]{.smallcaps} wihd)—it has no shape of its
own. Molecules in a liquid are farther apart than in a solid—they
don’t pull as hard on each other. So the molecules of a liquid can slide
around. That’s why liquids have no shape.
And air is a gas. It has no shape of its own, either. Its molecules
are so far apart that they hardly pull on each other at all. Molecules
of a gas bounce around so easily that they can squeeze into a balloon or
spread out to fill a room.
All the kinds of matter around us are solids or liquids or gases. So in
everything around us, molecules behave in certain ways. They hang
together tightly, slide arotmd each other, or move about freely in
space.
