Matter is all the stuff that we see, feel, and smell around us. By definition, matter has mass and takes up space, and it includes substances such as water, wood, rock, metal, plastic, and air, as well as countless others. At the most basic level, these substances are built from atoms, and the atoms, in turn, are made up of particles called protons, neutrons, and electrons.

Although atoms are too small to be seen, physicists, through experimentation and indirect observation have developed a good understanding of their structure. Unlike some early models of the atom, the current atomic model is loosely constructed. Surrounding a very small and dense nucleus of protons and neutrons is a "cloud" of electrons with orbits as large as 100,000 times the diameter of the atom's nucleus.

In addition to electrons, there is also a tremendous amount of space in the electron cloud. Even in a substance as hard as a diamond, there is far more empty space between the subatomic particles than there is space occupied by those particles. If this is so, how can any substance be solid?

The solidity of a substance is the result of strong electrical bonds between electrons and their nuclei. Despite their distance from the nucleus, the negatively charged electrons are held strongly in place by their attraction to the protons in the atom's center. This attraction creates a great deal of structural integrity and a strong force that repels the electrons of other atoms.

We benefit from this repellent force every day. Despite the empty spaces that exist in every atom, when we lean on a tabletop, our hands cannot pass through the wood or metal in that surface. Just as it would be impossible to pass one open fishnet through another open fishnet without cutting or tearing the fibers they're made of, it is also impossible to pass one atom through another without destroying the atoms entirely.