Sorry, I was not intending to start a big deal over this. While you are indeed correct that weight and mass can be used pretty much interchangeable in everyday life, technically there is a fundamental difference. Weight is defined to be equivalent to mass (the amount of matter in an object) times the acceleration due to gravity. This means that an object's mass never changes, but it's weight can, depending on the strength of gravity it is experiencing. Thus, on earth, since gravity doesn't change (much, but that's a whole 'nother can of worms), we can use them to refer to how much we are purchasing, etc., without it being an issue. Again, sorry for the trouble this caused.