With your palms flat, rise onto your toes into a high plank position. Keep your hands directly below your shoulders, your feet at hip-distance apart, and your core engaged.
Keeping your body in a straight line, bring your right knee to your right elbow. You should feel your obliques working.
Return your right foot back to your plank position, and repeat this on the left side, bringing your left knee to your left elbow and back to plank.
This counts as one rep. Continue alternating for 30 seconds.