Matter

Matter is the substance of objects — anything that has mass and takes up space. If you can weigh it and it occupies volume, it is matter. Rocks, water, air, and plasma are all matter in different states.

Matter exists in four commonly encountered states: solid, liquid, gas, and plasma. What determines the state is how tightly the constituent particles (atoms and molecules) are bound by forces. In a solid, particles are locked in place. In a liquid, they slide past each other. In a gas, they fly freely. In plasma, electrons are stripped from atoms entirely. Temperature and pressure govern transitions between states.

At the atomic level, matter is composed of atoms — protons, neutrons, and electrons — bound by electromagnetic and nuclear forces. Atoms combine into molecules, molecules into substances, substances into the things we encounter. The periodic table catalogs the elements: the distinct kinds of atoms, each defined by its number of protons.

Matter is what gives the physical world its resistance. You cannot walk through a wall because the matter in your body and the matter in the wall cannot occupy the same space at the same time (the Pauli exclusion principle, at bottom). Energy can exist without matter (a photon has energy but no mass), and matter can be converted into energy, but in everyday experience the two are distinct: matter is the stuff, energy is what the stuff does.