This is one of the most deceiving exercises of all time—it's much harder than it looks! You'll target your core, the backs of your arms, and your butt at the same time.
How to do it: Start kneeling, with knees directly below your hips. Brace your abs in tight and extend your left leg out straight. Your body should form a straight line from your left ankle to your head. Bend your elbows all the way down to the floor directly below your shoulders, palms facing down. Press back up, extending both arms straight, then quickly bend elbows back down to the ground. Repeat as many times as you can (be careful not to let your body rock back and forth as arms bend and extend, keep the motion up and down) for 30 seconds and then switch legs to repeat for 30 more seconds.