Behold the weirdest idea of modern science:
Sub-atomic particles do not behave like hard, tiny balls. True, they have position, mass and momentum. But particles may pass through one another just as waves of water pass through one another. Another tricky thing about particles is that we can never precisely predict the future location of a particle because we cannot observe both its position and its momentum at the same time. We can only make good guesses about where a particle is likely to be, so you may want to think of particles as "fuzzy regions."
Throughout this site we illustrate particles as shiny balls since balls are easy to draw. Please keep in mind that these really represent impossible -to-illustrate particle-waves!