What matter is

Everything around you belongs to one of two kinds: matter or energy.

Matter is the easier of the two to pin down, and it has a definition you can apply to anything you can point at.

A stone, the water in a glass, the air inside a balloon — all three are matter. Each takes up room, and each can be weighed. The air is worth pausing on: you cannot see it, but it fills the balloon and it makes the balloon heavier, so it passes both halves of the test.

Sunlight and the sound of a bell are not matter. They carry energy from one place to another, but they occupy no space of their own and they have no mass.

This whole chapter is about matter: what it is built from, and why one kind of matter behaves so differently from another.