What are protons made of?
Popular wisdom says they are made of three quarks: two up quarks and a down quark. However, reality is much messier: quarks are held together by the strong force, which is mediated by a particle called the gluon. The gluon sticks to everything: all the quarks and even itself! This makes the force so strong that quarks cannot escape: it is impossible to see a naked quark. In fact, the interaction created by the gluons is so strong that most of the mass inside the proton comes from them. In other words: protons are mostly gluons and then there are some quarks. Apart from gluons, you can find troves of other quarks and their anti-particles: however, they mostly “cancel out,” leaving a residue equal to the classical two ups and one down.
Read this excellent post by Prof. Matt Strassler to find out more.
Particle physics is messy. And gluons are sticky.