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Mass is "the amount of stuff" in an object. That is, the amount of atoms and molecules present.  Mass in science, is measured in the metric system using units such as kilogram (about 2.2 pounds)nwhich are a thousand grams, grams, centigrams (hundredths of grams), and milligrams  (thousandths of grams).  Mass is different from weight,  Weight is a force.  It depends on gravity pulling us toward the center of our planet (or another planet if we ever get there or the moon).  If we weighed ourselves on our bathroom scale and noted our weight and then took our scale with us to the moon, we would find that our weight was only one sixth as much there because the force of gravity is much smaller.  However, because when we find mass, we use a procedure that compares our object with known mass, our mass would not change regardless of the amount of pull of gravity.  This explains why astronauts did not change their size when they were on the moon even though their weight decreased considerably.  Well, OK, that's not exactly true.  Their size changed just a little. They were temporarily a tiny bit taller because gravity was not pulling them down, compressing their spines a little bit.  But that has nothing to do with the definition of mass.

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Mass is a property of matter, and its units are usually measured in grams (g) or kilograms (kg) worldwide and additionally ounces (oz) and pounds (lbs) in the US.  On the small scale, atomic mass, or the mass of elemental particles (protons, neutrons, electrons, and whole atoms) are measured relative to an atom of carbon-12, which is set to have a mass of 12.0000. Mass should not be confused with weight, which a measurement of how heavy matter is when subject to the force of gravity, so:

weight = (mass) * (force of gravity)  or w=mg.

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