is to clear the board of bubbles using a certain number of taps. each time a bubble bursts, it releases bubbles in four directions. when these bubbles hit other bubbles, they either pop those bubbles (which in turn release four more bubbles), or they turn those bubbles into the next color.
the bubbles come in four colors--red bubbles, which pop with one hit; green bubbles, which pop with two hits; yellow bubbles, which pop with three hits, and blue bubbles, which pop after four hits. when a green bubble is hit, it turns into a red bubble; when a yellow bubble is hit, it turns into a green bubble; and so 