While this bluffing card game won't prepare kids to be a spy, it gives them practice at keeping a poker face.
Each player gets seven cards, while the rest are turned facedown in a drawing pile. The dealer starts by laying a card or cards from his hand facedown on the table, then declaring their value (for example, "three sevens").
The next player has to add a card or cards of the next highest value (in this case, eights). If he or she has no such card, the choice is either to pick from the drawing pile -- or to fake it.
Any player can challenge by saying, "Lie detector," but when the truth is revealed, whoever is wrong inherits all the cards in the facedown pile. First one to get rid of all their cards wins.