When you make special combinations, such as four or five in a row, you will earn various special potions. These will have a special effect when you tap them such as knocking off one line, or a 3x3 box of soaps. Mix two purple potions together and you'll get a rainbow potion, which you can tap for even bigger effects, such as knocking off 3 lines or blowing up a 5x5 area of soaps.