Math Stacker requires the player to use the touch input feature of the iPod Touch to determine the equivalence relationships among fractions, decimals, and percentage values and use it to sort a list of such values. First, a practice example is provided to help the player to learn the interface and interaction technique. Then, a timed exercise (30 seconds) challenges the player to sort as many lists of numbers as possible. Both a timer and color is used to indicate the time remaining (green - more than 10 seconds, yellow - between 5 and 10 seconds, red - less than 5 seconds). Once the displayed list is properly sorted, a new list is generated. The list contains decimals, fractions and percentage values. The sorting is achieved through a sequence of numbers swapping. Two numbers are swapped by touching one by one the numbers in the pair.