Click a square to lay the next pipe from the queue — you can see what's coming but not reorder it. The bar along the bottom counts down to the water starting at the green source. Get the flow through enough pipes to clear the level — then keep laying ahead of the water, because every pipe past the target is worth bonus points and the level only ends when the water stops. You can freely replace a pipe the water hasn't reached, and the cross piece carries water both ways.