This exercise might remind you of Jane Fonda's workout videos from the '80s, but it's been a staple of the Pilates mat repertoire for decades. Since the knee is straight, you work all the muscles of the inner-thigh group.
Lying on your side, lengthen your bottom leg and cross your top leg over it. Rest either your knee or foot on the floor. Prop your head up with your hand, or rest your head on your arm.
As you exhale, lift your bottom leg up, and inhale as you lower it back down. Your torso should stay still while you do this.
Do 10 reps, then repeat on the other side.
Watch a video on how to do the side-lying leg lift here.