We're made of flesh and bone and fat, which are in turn made of protons and electrons, which are (mostly) made of quarks. Which, even though they are the most basic form of matter, are a minuscule percentage of your body's mass. Wait, what? Why do we have so much more mass than what we're made of?
Image of quarks from the film The Flow by Markos R. Kay
Step on a scale, look down, and divide the number you see by one hundred. If you were reduced to your existing quarks, on their own, piled up on the scale, that is how much those quarks would weigh. While weight isn't the same as mass, weight can be used to measure mass, and your mass, in terms of the actual bits that make up the nuclei in your atoms, is much smaller than it appears.
It's doubtful that anyone would be able to get a pile of single quarks, as quarks don't travel alone. When they make up a nucleon - a proton or a neutron - they come in threes, held together by gluons. The quarks, combined, have one percent of the mass of the nucleon, and the gluons have no mass at all. So ninety-nine percent of the mass in every nucleus is the energy pushing the quarks together.
You can see a visualization of how atoms and quarks work inside a cell nucleus in this gorgeous film, The Flow, by Markos R. Kay:
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.
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.
But all forms of resonance add mass. Simply put, the position of the quarks, and the forces acting on them, are part of the mass of the nucleon. They are the majority of the mass of the nucleon.
This "energy-is-mass" may sound like a technicality, but it's not confined to quarks. There is a somewhat larger-scale and somewhat more intuitive example. During fission, a uranium atom splits into two smaller atoms. This liberates a massive amount of energy, and the resulting smaller atoms have less mass than the larger uranium atom. (Roughly speaking, the fission reaction resulted in a 0.9 percent loss of total mass for the system.) But where does the mass-energy come from? Examine the products of fission, and you'll find that there is no proton or neutron missing. There is no neutron or proton that has less mass than it did before the split.
Uranium is a big, unwieldy atom. A lot of energy gets sunk into keeping its nucleus together. After the uranium atom splits, the resulting smaller atoms take less energy to keep together. It was the energy keeping uranium together that contributed to its mass. Mass, even at this level, is the result of the energetic reactions between the particles.
Top Image: National Human Genome Research Institute