Why does relativity mean that mass changes with speed?

Everyone knows that relativity plays hell with time, and that it can do a number on space, but what about mass? Why do objects get more or less mass depending on their relative speed? We're going to give you a quick explanation of why running can make you gain weight. No, we don't think it's fair either. » 1/09/13 11:19am 1/09/13 11:19am