This total-body move really zeros in on your core muscles, which not only include your abs, but also your back, hip flexors, and inner thighs.
How to do it: Start lying on your right side and bend your right elbow underneath your shoulder to prop up your body, reaching your left arm straight up to the ceiling. Bend your right knee and keep your left leg extended, with your foot flat on the floor. Press your hips off the ground and lift your right knee off the floor and bring the inside of your right foot to the left inner thigh. Next, turn your right knee up towards the ceiling (keeping your foot still pressed on inner thigh), tracing a ‘circle’ with your knee. Make 10 circles with your right knee, and then switch to the other side and repeat with your left.
Bonus tip: This move is tough, so if you need to, just work on holding your side plank position with your knee off the floor first and then build up to tracing the circles.