Matter is a substance that has inertia and occupies physical space. According to modern physics, matter consists of various types of particles, each with mass and size. The most familiar examples of material particles are the electron, the proton and the neutron. Combinations of these particles form atoms.
Solids, liquids and gases are called the three states of matter. Materials can be changed from one state to another by heating or cooling. Water can be observed as a liquid, a solid (ice), or a gas (water vapour) and moves around the environment in a process known as the water cycle.