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Mass is a measure of all the matter that makes up an object, and weight how much gravity is acting upon an object. Since we're on Earth and used to measuring things on Earth with Earth's gravity, if your mass is 50 kg, we say that you weigh 50kg. A scale on Earth would read 50kg, or 110lbs. On Mars, there is less gravity to pull on your mass, so you would only weigh 18 kg, or 40lbs. Your mass, however, is still 50kg, because that is the amount of matter that makes you up, no matter what.