5 Hugely Fun Facts About Mass (Not Weight)

A tiny leaf cutter ant carrying a huge leaf in his mouth while zipping along the edge of a sidewalk in the Soberania National Forest in Panama. Mass and weight indeed. avid_creative/Getty Images

Climb Mount Everest and you're guaranteed to lose some weight, but not because of the exercise. A person who's standing at sea level weighs slightly more than they would atop the mountain's peak. Weight is the measurement of gravity's pull on an object. And it varies by location.

Mass is a different beast altogether, one that's proven extremely hard to define. Although this is a bit of an oversimplification, in physics classrooms, students are told mass denotes two key characteristics of an object. The first is the amount of matter it contains. The second is the thing's ability to resist changes in its state of motion. (We call that phenomenon "inertia.") Unlike weight, mass is constant and holds firm no matter where an object travels.

You can never know too much about this all-important property; here are five massively cool tidbits we felt like sharing.

1. There's a Unit of Mass Called a 'Slug'

It's a part of the U.S. Customary Units system and the less popular British Imperial System. For most of the world, the preferred unit of mass is the kilogram, one thousand of which equal a metric ton. Kilograms belong to the International System of Units, also known as the metric system. While they're everyday terms in other countries, Americans tend to be more reliant on U.S. Customary Units.

Now you might assume this system's answer to the kilogram is the pound. Yet pounds are technically units of weight. Both the U.S. Customary and British Imperial Systems measure mass with a different unit called a "slug." (On Earth, one slug is equal to about 32.2 pounds, or 14.60 kilograms.) Even so, it rarely comes up in casual conversations and most users aren't familiar with the term. That's a real pity; imagine WrestleMania fans cracking slug jokes by the ringside.

2. The Scientist Who Discovered the Law of Conservation of Mass Was Beheaded

"In every operation," wrote the great chemist Antoine-Laurent Lavoisier, "an equal quantity of matter exists before and after the operation." Put another way, mass can neither be created nor destroyed. This principle has been named the Law of Conservation of Mass. Lavoisier's experiments in the late 18th century brought this idea to light.

Fellow scientists embraced his findings, but Lavoisier's career was cut short. Literally. When he wasn't decomposing water or making rust on purpose, Lavoisier helped collect taxes for the French government. That got him guillotined in 1794, after he was charged with "conspiracy against the people of France" by revolutionary forces.

3. It's Part of the Most Famous Equation Ever Written

Of course, we're talking about E = mc2. Stated in plain English, it says energy (E) equals mass (m) times the speed of light (c) squared. Albert Einstein discussed the flipside of this equation in a classic paper published Sept. 27, 1905. By the way, he was only 26 years old at the time.

Said Einstein, "It followed from the special theory of relativity that mass and energy are both but different manifestations of the same thing — a somewhat unfamiliar conception for the average mind."

So there's an inherent energy found in all objects that possesses mass. Einstein's breakthrough explains why every atom is slightly less massive than the sum of its parts (namely the protons, neutrons and electrons which comprise it). And the same energy/mass relationship he observed accounts for the destructive power of atomic bombs.

4. Light Consists of "Massless Particles"

Photons are the fundamental particles of light. Experts describe them as being "massless." You see, the speed of a traveling object always changes its mass. Because that can complicate scientific discussions, when physicists talk about the mass of a given body or particle, what they're usually referring to is its rest mass. Basically, that's the mass it possesses when its velocity is equal to zero. Neutrons, protons and electrons all have rest masses — but photons don't! Neither do gluons, another type of subatomic particle.

5. Earth Shares a Common "Center of Mass" With the Moon

Earth's mass is 81 times greater than the moon's; the disparity has a profound effect on their relationship. When you've got two or more heavenly bodies — like moons, planets and suns — orbiting each other, they're really revolving around a common center of mass. Called the barycenter, its location depends on the participants.

If two objects with the exact same mass start orbiting one another, their barycenter will be situated directly between them. But since Earth is so much larger than the moon, the Earth-moon barycenter is located deep inside our home world. And yet the Earth still revolves around it, just like the moon does.