What is a "particle"?

Quantum physics says everything is made of particles, but what does that actually mean? "Is he a dot or is he a speck? When he's underwater, does he get wet? Or does the water get him instead? Nobody knows." --They Might Be Giants, "Particle Man" We learn in school that matter is made of atoms and that atoms are made of smaller ingredients: protons, neutrons and electrons. Protons and neutrons are made of quarks, but electrons aren't. As far as we can tell, quarks and electrons are fundamental p...