Solids are the state of matter that holds a definite shape and takes up a definite amount of space. You can describe any solid by its properties: how hard it is, whether it bends or breaks, how heavy it feels, its color, and its texture. Different solids have different combinations of these properties, which is why we pick certain materials for certain jobs — a hammer head is hard metal, not soft clay.
Set up a "properties station" where students handle a collection of solids — a wooden block, a rubber ball, a metal spoon, a sponge, a rock, and a piece of chalk. Have them fill in a chart describing each object's hardness, flexibility, texture, and whether it floats or sinks.
You already know that solids keep their own shape and take up a definite amount of space. But saying something is "a solid" is just the beginning — there are thousands of different solids, and they can be wildly different from each other. A diamond and a marshmallow are both solids, but you would never confuse them. The way we tell solids apart is by describing their properties.
Hardness is one of the most useful properties to notice. A rock is hard — you cannot dent it with your fingernail. A piece of clay is soft — you can squish it easily. Hardness tells you how well a solid resists being scratched or dented. Scientists even have a scale for this, but at your level, the fingernail test works great: if your fingernail can scratch it, it is pretty soft; if it cannot, the solid is harder than your nail.
Flexibility tells you whether a solid bends or snaps. A pipe cleaner bends easily and stays bent — that is flexible. A dry stick might bend a little, then snap — that is brittle. A steel beam barely bends at all — that is rigid. Notice that all three are still solids. Bending does not turn a solid into a liquid. The solid is still holding a shape; you are just changing which shape it holds.
Other properties you can observe include texture (smooth, rough, bumpy, fuzzy), color, weight (how heavy it feels), and whether it is transparent (see-through) or opaque (blocks light). Every solid has its own combination of these properties, like a fingerprint. When engineers choose a material to build something — a bridge, a toy, a phone case — they match the properties of the solid to the job it needs to do.
Try this at home: pick up five different solid objects and describe each one using at least three properties. You will quickly see that no two solids are exactly alike, and that is what makes the world of materials so interesting.