How to do it: Grab a pair of dumbbells and stand facing a sturdy, high box or step. Step your right foot onto the box, bring your left leg further behind you on the floor in preparation for a lunge. Bend both knees and lower into a lunge position. As you come up out of your lunge, stand all the way onto your right leg, stepping up onto the box, and tap your left foot on the box. Bring your left leg behind you and lower back to your start position. That's one rep. Do 15 reps on the right leg, 15 reps on the left leg.
You can make this easier by doing the lunge without added weight. To make it harder, lift your knee (instead of tapping your toe) as you step onto the box.