Do you remember the story of Tom Sawyer? At one time, he was required to whitewash a fence. He cunningly recruited all his friends to do the work by telling them it was fun, and offering to let them do it for a small fee.
Basically, you are presented with an image. You try to give it a meaningful label. If you and another person both choose the same label, you move on to a new image and you both get points. The top scorers are posted on google.Very interesting, and the funny part is, I think that doing this work for google (in the form of a game) would be kind of fun!