The Riddler

Answer

Answer: When he picked out a grape (purple, remember that both grapes were purple) he ate it, moving his hand to cover the color of the grape in a fast motion. Then, he picked the remaining grape out of the bag, showing it to everyone. Then, he announced,"This grape is purple. Therefore, that means the grape I have devoured is green." The king looked embarrassed, and back in the castle he apologized to the prince.