They can also be made up of mixtures, or combinations, of different elements and compounds. Most of the solids you see are mixtures.
Most rocks are mixtures of many elements and compounds.
Concrete is a good example of a man-made solid mixture. First let's explain that characteristics are the traits or features that something might have. So one characteristic of a solid is that it might be hard. Pretty straight forward. One of the main characteristics of solids is that they hold their own shape.