Combos are earned when you pop at least 2 or more water bubbles with a single move. For example, if there are two 6's, then put down two tiles that equal 6 (e.g., 2 + 4). This way both 6's will pop simultaneously, giving you bonus points and will evaporate so the tank won't fill up so fast. Try to strategize such that more than one of the same number becomes active before you target it (when possible). Another example: If there are three 10's, then place three tiles that equal 10 (e.g., 5 + 3 + 2). This will result in a triple combo!