I recently read about a grandfather who had a dairy farm and convinced his visiting grandchildren this was true. He secretly added some chocolate powder to the bottom of a pail and then let the children watch him milk a brown cow into the pail. And sure enough, the pail filled up with chocolate milk, which the kids got to drink.
The letter I read was from one of the now-adult grandchildren, who admits he believed for years afterwards that it was possible to get chocolate milk from a cow - he had, after all, personally witnessed it happen.