You could say that some of this energy translates into physical mass. After all, pry quarks apart and you don't get lone quarks, you get a host of extra particles. But it's not that simple. Those extra particles, added together, don't add up to the mass of the original three bound-together quarks.

#### The Energy and the Mass

Put particles together in a very tight space, and they zoom around faster and faster. This kinetic energy applies to quarks. Bound together in one nucleon, these quarks have a great amount of kinetic energy. This translates to inertia. And inertia, as Newton's laws of motion let us know, is a property of mass. Put it in motion or at rest, it stays that way until a force comes along to move it.

But kinetic energy isn't the only thing that bulks up a nucleon. The so-called "color-force" or "strong interaction" of quarks exerts a curious property called "confinement." The confinement can force the quarks to interact with each other in ways that add to the energy, and so add to the mass. The force itself acts like a rubber band. Stretch it too much and yes, it snaps. Stretch it a little, though, and it just squeezes down harder. This leads to a kind of super-energetic state inside the nucleon. And this state, called resonance, adds to the mass of the squished-together quarks. The most famous of these states is called the Delta resonance. It takes a lot of energy to produce this resonance, which is why we are relatively light. Want to increase your weight by one-third? Just kick all your quarks into Delta resonance.