Chemical refers to any substance made of matter, especially when we're thinking about what that substance is made of at the smallest level or how it might react with other substances. Water is a chemical. Salt is a chemical. The oxygen you breathe is a chemical. Even your own body is made of thousands of different chemicals working together.

Scientists use the word chemical when they want to be precise about what something is made of. A chemical reaction happens when substances interact and transform into something new, like when baking soda and vinegar fizz together, or when iron rusts. Understanding chemicals and how they behave is the work of chemistry, one of the main branches of science.

Sometimes people use chemical to mean something artificial or potentially harmful, as in “I don't want chemicals in my food.” But this isn't quite accurate, since everything, including the most natural foods, is made of chemicals. What they usually mean is synthetic chemicals added during manufacturing.

A chemist is a scientist who studies chemicals and how they interact. Modern medicine, materials like plastics and steel, and even the colors in your clothes all exist because chemists figured out how to work with chemicals in useful ways.