Mountain climbers

Muscles worked: abs, obliques, quads, hamstrings, deltoids, biceps, triceps, and the chest.
Start off in the standard push up position. Pull your knees up to your chest, one at a time, in quick succession. You want to keep your body in a push-up position all the way through the exercise, so don't bob your hips up and down as you're tucking your legs in.  Do 20 seconds three times.