And in fact, atoms are mostly composed of empty space. And in fact, if our atom with the size of a football field, our nucleus will be the size of a marble. So you can imagine that our entire atom, for the most part, is composed of empty space. And that's because our electrons are very, very small and they orbit our nucleus at a very, very great distance compared to the size of the nucleus itself. So what holds the nucleus or the protons and neutrons together? Well, the force that holds the nucleus together is called a nuclear force.