Despite many repeated experiments with trillions of protons, we have never, ever observed the decay of a single proton into it's constituent quarks. They seem to be the most solid bit of matter in the Universe with a half life of some number longer than the life of the Universe. A proton and neutron are each made of 3 quarks (2 up one down and 1 up 2 down respectively) but outside of the nucleus, the a neutron has a half life of 10 minutes. Amazing, no? Stars blow up and fade, atoms come and go, photons wink in and out, galaxies end, butter left on the counter overnight melts --- but protons are forever.