>
> > Since it's known how much energy is in a gallon of gas
>
> even if it is not known, you can measure it with a calorimeter: You make a thermally isolated chamber, put some thermometers in it, put a match to your gasoline (or petrol :-) or whatever) and measure how much the temperature goes up when it burns. This temperature difference converts directly to energy.

You could have just told them it's called a "bomb calorimeter".

There. Isn't it amazing how just *one* word can make a process as infernally boring as "calculate the number of BTU's generated" suddenly that much more interesting? :)