Start lying on your back with both legs extended toward the ceiling, pressing your lower back into the floor. Rest your arms on the floor next to your body.
Cross your right ankle over your left, and lower both legs toward the floor. Stop when you feel your lower back begin to lift off the floor.
Uncross your ankles and cross them with the left ankle on top, then raise your legs back up toward the ceiling. That's one rep. Begin the next rep with your left ankle still crossed on top.