Unlimited air.
You control a dolphin carrying a cat along a path from start to finish. Pressing left mouse button speeds up the dolphin, releasing the button slows down the dolphin and releases the cat from the dolphins back. That's all the controls. Here's the challenge: while in the water, the cat's air supply decreases. So you must release the cat at peak places so that so that the physics sends it on a trajectory upwards above the water. Next you must time the dolphins speed so that you catch the cat on its way down. This sounds simple but proves to be really tricky at times.
the air is running out! Toss the cat!