This excellent core exercise is sort of a combination of an old-fashioned sit-up and a leg lift. To get into the start position for this one, lie on the floor with your head resting on the ground, your legs straight out in front of you feet just a few inches apart from one another.
Your arms should be straight up over your head, also flat against the ground. To begin, simultaneously raise your legs up in the air towards the ceiling while raising your upper body up off the ground (like a sit-up) bringing your arms up over your head like you're trying to touch your toes up in the air.
When you reach the top your legs and arms should in an inverted 'v' shape. Pause for a one count at the top, return to the starting position and repeat. Keep your legs as straight as possible as you raise and lower them. Keep your back straight throughout the movement.