Best gag I ever did was when I was a kid, next door neighbor rebuilt an old car including the engine completely and did a great job, he checked things like gas mileage constantly and then would spend the week bragging to my dad about how great it was running and how good the mileage was. I'd go over there at night and siphon out five gallons, he would spend the following weekend tuning and tinkering. About Sunday night I'd go pour the 5 gallons back in it. Then the bragging all week again. Back at night and take out 5 gallons and another weekend spent tinkering and then the bragging again. That went on every week for a month and a half, he never did catch on.
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