My favorite is still from a time at a gas station, while filling the tank, a little boy came up. "Excuse me". I looked over at him "Yes sir, what can I do for you?" He pointed to the wind-up key I had on the car at the time, and asked if I had to use that to make the car go. Trying not to laugh out loud (don't want to discourage him from asking in the future), I answered "No, it is just for fun.". He promptly turned to face across to another pump, and yelled "No Mom, it is just for fun!"
Busted!