Some levels contain bubbles that float up from nowhere. There are some that pop all bubbles on the screen when you burst one. If you blow a bubble at that type of bubble, it changes color. Do it again, and it becomes one of the letters to spell "EXTEND". Using all possible combinations of the three colors, you will eventually spell the entire word, earning an extra life and skipping the current level. The combos are as follows.
Red + Red = E
Red + Green = X
Green + Green = T
Green + Blue = E
Blue + Blue = N
Blue + Red = D