Kakuro is a kind of logic puzzle that is often referred to as a mathematical transliteration of the crossword. Each "word" must add up to the number provided in the clue above it or to the left. Words can only use the numbers 1 through 9, and a given number can only be used once in a word. The aim of the game is to fill all the blank squares in the grid so that the numbers you enter add up to the corresponding clues. Great app for a math geek!