This is a great exercise to work your lower abs and can easily be done anywhere. All you need is an exercise mat.
Lay with your back flat on the mat and be sure to keep it that way through out the movement of this exercise. It’s very important that you DO NOT ARCH YOUR BACK. You can either lay your hands at your sides or over your head, which ever is more comfortable to you. Keeping your legs straight (a slight bend at the knee is okay) swing them up until your feet are pointing straight above you, then slowly release them back toward the mat, but don’t touch your heels to it. Your feet should be close to touching, but hover a couple inches from the floor. If you find that your back is arching, only go half way up and down until you build enough strength in your core to do it the full 90 degrees.