Materials are the 'stuff' the world is made up of. Materials are all different. Some are strong, or hard, or transparent. Some are weak, or soft, or opaque.

We pick a material to do a job because it will be good at it. Windows would be no good if you couldn't see through them. A chair would be useless if it collapsed when you sat on it.

So we choose materials that are right for the job. We may shape or change them. We may even make new materials that will fit the task. We test materials to make sure that they are strong enough, or bendy enough, or absorbent enough, for the job.