Becky, a student pilot in Florida, writes: I’m fully caffeinated and working my way through Chapter 10 of the Pilot’s Handbook of Aeronautical Knowledge to get my head around this weight and balance stuff. I just encountered “standard weights.” The good book says that standard weights are “established weights” and “should be used if actual weights are not available.” Then the first thing on the list is gasoline at 6 pounds per gallon. Huh? How can the weight of gas be a mystery? It weighs what it weighs…right?