Particles work more or less like matryoshka dolls, the wooden dolls that can be opened and contain smaller and smaller dolls inside. The smallest one is a whole piece, not openable like an elementary particle, also called a fundamental particle.

In Book 8, Millie tries to break an electron, but there’s no way because the electron is an elementary particle; it’s a whole piece like the smallest doll. Physicists would say it has no internal structure.

The nucleus of the atom is not an elementary particle because it contains even smaller particles: protons and neutrons. The nucleus is like one of the opening dolls, or like the shell of a chocolate egg, and the surprise, neutrons and protons, correspond to its internal structure.

Neither protons nor neutrons are fundamental particles because they are themselves shells and contain as internal structure the quarks.

Quarks are elementary or fundamental particles, like electrons.