What is a solid?
Have you heard the phrase "as solid as a rock"? It is used to describe firmness and strength. The firmness of a solid is due to one of its special properties. It has a fixed shape. This means that if you leave a solid alone, its shape will not change. For example, a piece of rock in the shape of a cube does not squash down to the shape of a pancake when you leave it alone. It remains a cube. The shape of a solid can be changed but you have to push or pull on it. The shapes of some solids, such as Plasticine, are easy to change. When you have finished working on them, they stay in the new shape. There are exceptions to this. If you pull an elastic band, it becomes longer and thinner. When you let it go, the elastic band springs back to its original shape. Some solids are so resistant to pushes and pulls that they break rather than bend. For example, you can bend a piece of metal wire and it will take and hold a new shape but when you bend a piece of wood, like a matchstick, it snaps.

The shape of a solid can be changed by cutting it. A carrot can be easily cut up into slices to make cooking easier. We cut up our food before we eat it so that it is easier to digest. The wood that snaps when we bend it can be cut with a chisel and carved into complicated and beautiful shapes such as those found on old chairs and tables.

Although solids may differ in the way they behave when pushed, pulled and cut they all have one common property. They cannot be squashed. When you try and squash something you try and get it into a smaller space. A solid will just not get any smaller no matter how hard you try to squash it. Some solids may crumble to pieces when you squash them but their volume will stay the same. If you were to stick all the pieces together you would see that they occupied the same space as the original piece of solid.

What kinds of materials are solids?
Most kinds of materials are solids. They include rock, pottery, metals, wood, paper, cloth and plastics. Food such as cheese, butter, potatoes and apples are solids too.

How can butter and metal be put in the same group - solid materials?
They are both solids because they both have a fixed shape and cannot be squashed to fill a smaller space. If a substance has these two properties it is a member of the group of materials called solids.

What is the difference between a solid and a hollow object?
A solid object is made entirely from one or more solid materials. A hollow object is made from a solid material which has a space filled with air or a liquid. For example, an empty jar is a hollow object. Its walls and base are made from glass and the inside is full of air. The hollow part of the jar may be filled with a liquid such as water or oil. Some of the rocks near a volcano have hollows in them. In the past the hollows were filled with liquids that cooled to form crystals. Today there is an air space in the middle of the hollow and inside walls of the rock are lined with crystals. These hollow rocks are called geodes.

A sponge is full of holes. Is it a solid?
Yes, it is. The substance that is used to make most sponges is plastic. This is a solid. It just happens to have a shape with lots of holes in it. The holes are filled with air. When you press on a sponge you can squash it. This happens because you push the air out of the holes and the strands of plastic come close together. You squash the object called the sponge because the air escapes from it but you do not squash the solid substance called the plastic. The strands just lie closely together but do not get any smaller. When you stop squashing the sponge, the springy property of the plastic makes the sponge go back to its original shape and fill with air again.

How do scientists know that solids are made of particles that are stuck together?
Have you seen an X-ray photograph? They are used to show the inside of a person's body. Scientists have also taken X-ray photographs of solid objects such as crystals. When the scientists looked at the photographs they found that the X-rays made regular patterns on the film. The only way to explain this was to imagine that tiny particles were arranged in rows inside the solid. Later, very powerful microscopes were invented. They are called electron microscopes. When scientists used electron microscopes to look at solids they actually saw the particles arranged as they had imagined.

Can a solid change into anything else?
Yes. When many solids are heated they become softer. Eventually they become so soft that they can no longer keep their fixed shape. When this happens, the solid becomes a liquid. The process of solid turning into a liquid is called melting. The temperature at which a solid melts is called its melting point. Some solids, such as wood, do not melt when they get hot. They catch fire and burn to make ash and smoke.