Someone once mentioned that their parents told them that the ice cream van played music to let you know that it had sold out of ice cream so you wouldn't waste time flagging it down. So when our (little) kids asked why that van was playing music, we couldn't resist (could barely keep straight faces either)..... a couple of years later middle son pipes up "look they just got ice cream from that van, they mustn't know the music means it hasn't got any!"
Never thought they'd believe us. Now they're just really unsure about the whole thing. We are bad, bad parents.