quark

If you keep cutting matter into smaller and smaller pieces — a brick into grains, a grain into molecules, a molecule into atoms — you eventually reach the nucleus, made of protons and neutrons. For a long time those looked like the end of the line. They are not. A proton or neutron is itself built from even tinier specks called quarks. As far as anyone can tell, a quark has no smaller parts at all: it is one of the genuinely fundamental building blocks of matter.

Quarks come in six types, called flavors, but everyday matter uses only the two lightest: the up quark and the down quark. A proton is two ups and one down; a neutron is two downs and one up. Quarks carry electric charge in odd fractions — the up has +2/3 of a proton's charge, the down has −1/3 — and the fractions add up exactly so that a proton comes out at +1 and a neutron at 0. Quarks also carry a second kind of charge, called color, which has nothing to do with visible color and is the source of the strong force that glues them together.

The strange thing about quarks is that you can never pull one out and hold it on its own. Try to yank a quark free from a proton and the strong force between it and its neighbors does not fade — it stays strong, so the energy you pump in eventually conjures up new quarks, and you end up with more particles rather than a lone quark. This locking-away is called color confinement, and it is why no one has ever caught a single bare quark. We know quarks are real mainly from firing high-energy probes into protons and seeing the beams scatter as if they hit three hard little lumps inside.