I made this up because my students needed extra practice with missing addends. This is kind of like the game Bang! that I use for sight words. I use it in small math groups of about six students. I show the card to each student and they have to come up with the missing digit quickly. I provide them with a number line. If they get the correct number, they get to keep the card. Then if I pull a POW! Card, they have to give me all of their cards and they start empty. Then I just have them count the number of cards they got in the end. Technically, the winner is the person with the most cards. My second graders LOVED this game and it really helped with this tricky skill.