http://www.kiel.k12.wi.us/faculty/jwills/Concentration.cfm
You need to put tagboard or other stuff paper in your inkjet printer to print these out, but print the fact cards and the matching answer cards, then cut them out. Shuffle and spread them out on a table face down. Player A turns over two cards; if they’re a match then they remove those two cards and take another turn. If they aren’t a match, Player A must turn them back over in the same place, and it’s Player B’s turn. The winning Player is the one who has the most pairs of cards when all have been turned over.