CoachA solid is stuff that keeps its own shape. A key keeps its zigzag in your pocket, in a drawer, or underwater. You can hold a solid between two fingers, and moving it does not change what it looks like. Some solids are hard (stone), some soft (bread), some bendy (a rubber band) — but left alone, each keeps its shape.
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Example 14.2 (A solid made of grains)
Sand seems to break the rule: poured from a bucket, it takes the shape of its pile, almost like a liquid. But look closely: sand is a crowd of tiny solid grains, and each grain keeps its shape perfectly. Flour, sugar and rice play the same trick — crowds of little solids that flow together while each member stays solid.