If you want to frustrate yourself when it comes to exercises then this is the perfect one for you. The aim is to keepthe bottle of water completely still. This is measured by whether or not there is any movement on the surface of the water within the bottle (which should be half full). So, lay on your back on the floor, grip the fit-ball between your ankles and lift it with a 90 degree angle at the knee, and your lower legs parallel to the floor. Whilst you do this you hold your upper body in a straight crunch position, so you are activating both your upper and lower abdominals. Get a friend/trainer to place  the bottle of water on the top of the fitball and look very closely at it. I bet you can’t keep it completely still for a period of 5 seconds. Good luck, let me know how it goes. P.S. I can’t do it !!