color confinement
Take a bar magnet and snap it in half, and you do not get a lone north pole and a lone south pole — you get two smaller complete magnets, each with both poles. Quarks behave in a similar, even more absolute way. No matter how hard you try, you can never isolate a single quark. Pull one out of a proton and you do not end up holding a free quark; you end up making new particles instead. This rule is called color confinement.
The reason lies in how the strong force behaves with distance. For electricity, the pull between two charges fades as you separate them. For the strong force it is the opposite: as two quarks move apart, the gluons between them bundle into a taut tube, and the force stays roughly constant — like an unbreakable rubber band that stores more and more energy the more you stretch it. Eventually so much energy is stored that it becomes cheaper for nature to convert that energy (via E = mc²) into a fresh quark-antiquark pair. The string snaps, but the two new ends are capped with new quarks, so you simply end up with two colorless particles instead of one. A bare, colored quark never appears.
Confinement is why everything we ever detect is colorless: protons, neutrons, pions, and all other hadrons are color-neutral bundles. It also means the quarks and gluons inside a proton are permanently hidden from direct view. Curiously, confinement has never been proven from QCD's equations with full mathematical rigor — it is overwhelmingly supported by experiment and by lattice simulations, but a clean proof remains one of the famous open problems in mathematical physics.
Smash an electron into a proton hard enough to knock a quark loose, and instead of a free quark you see a narrow spray of ordinary hadrons — a jet — as the snapping color string makes new particles.
Trying to free a quark produces jets of new hadrons, never a bare quark.
At extreme temperatures (like the early universe or heavy-ion collisions), quarks and gluons do briefly roam free in a quark-gluon plasma — confinement is a property of ordinary cold matter, not an absolute law for all conditions.