Every object in the universe has mass. It is the amount of matter an object contains and it never changes, no matter where that object happens to be. We often say that objects with a large mass are heavy and that objects with a small mass are light. This is true, at least in the situations we deal with here on Earth. However, mass and weight are not the same thing. Unlike mass, the weight of an object is simply a measure of how hard gravity happens to be pulling on that object. Because Newton's law says that the gravitational attraction between two objects is proportional to the product of their masses, the more mass an object has, the harder Earth's gravity will pull on it.
Another way in which weight differs from mass is that the weight of an object can actually change depending upon where it is. Because Newton's law says that the gravitational attraction between two objects decreases the greater the distance is between them, an object will get lighter the farther away it is from the Earth (or from any large body). This means that you will actually weigh less standing at the top of a high mountain than you will at sea level! Of course the difference is incredibly small but it is detectable.
For an object to really lose weight it must be far away from any large bodies like planets or stars (in deep space for example). When an object is far enough away from these bodies that it experiences practically no gravitational pull from them, it enters a state of zero gravity or weightlessness. As its name implies, an object in a weightless environment weighs nothing, no matter how large its mass.
For more on Newton's law of universal gravitation see What is Gravity Page 4.
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