Matter is all the stuff that we see, feel, and smell around us. By definition, matter has mass and takes up space. Matter includes substances such as water, wood, rock, metal, plastic, and air, as well as countless other materials. All types of matter are made up of tiny particles called atoms. Atoms and the sub-atomic particles they're made of are far too small to be seen, even with the most powerful microscope. Despite this, physicists have developed an understanding of the structure of these particles through experimentation and indirect observation.