The simplest definition of matter is that it is anything that has weight or fills space. Matter is all the things we can see and feel in the world. Rocks, giraffes, water, and cars all consist of matter, whether they are living or non-living. Air is also matter. Though it is invisible, its effects can be felt, as when the wind blows. And with careful measurements, its weight can be calculated. Matter is often broken down into three categories: solid, liquid, and gaseous. For biologists, such a basic description of matter often suffices. But as long ago as ancient Greece, scientists theorized about what matter is comprised of. And in terms of modern-day physics and chemistry, our understanding of matter has grown quite complex.

Many of the renowned Greek philosophers discussed matter. Empedocles (c. 495-435 B.C.) believed all matter was made of a combination of four elements...