Step 1: Sit close to the edge of a sturdy chair (one without wheels, if possible).
Step 2: Place your hands on the sides of your chair or on the armrests.
Step 3: While keeping your back straight, lift your knees. Bring them up to your chest and then back down.
Step 4: You can let your legs go all the way to the floor during the exercise, or not allow them to touch the ground between reps to make it even more challenging.