Mass, you people are talking about the mass of an object, which doesn't change. Weight is generally a measure of force exerted by an object due to gravity. Weight generally falls under the auspices of f=ma where f equals the force measured (weight in this case), m equals the mass of an object and a equals the acceleration due to gravity. Weight (the measure of force) is also influenced by the medium in which an object is weighed, even air has an effect. Also, although we generally consider the acceleration of gravity to be a constant, it does vary dependant upon your location on the earth. This variation is very small fortunately. The mass of an object is constant no matter where an object is weighed.
A rock, however, will weigh less in the water than on land.
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