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This exercise should only be tried when you are flexible. You may need to follow a regime of improving your flexibility first, otherwise you will find this exercise next to impossible.  You can do this by jogging, doing jumping jacks or running on the spot for at least 10 minutes.  Extend your right leg straight out in front of you. Gently stretch for 10 seconds. Switch over to do the same with your left leg. Stretch for 10 seconds. At this point, you may find that it isn't happening as well as you'd like. That's fine––take all the time you need and keep working on this step until you get it. Put both hands in the middle of your legs, and lean down. Stretch for 15 seconds. Your legs should form a butterfly shape. Do this gently. Lean to the right for 20 seconds, then lean to the left for 20 seconds. Have them above your head and keep them up for 30 seconds. You can choose to stop here if wished, as your legs are now over your head. Or, you can go one further with the next step. When you have achieved this, then try to bring back your left leg as well. At this point, realize that this is exceptionally hard to do, so do not push it. It can take a very long time to do this successfully, perhaps even a year or two before it works for you. The second foot is harder to do than the first, because you have to get it beyond the first. Whichever leg is less flexible, do first.

Summary:
Work on your flexibility. Place a mat on the floor where you intend to practice this exercise. Warm up your muscles first. Stand up and stretch for 15 seconds. Sit down on the floor mat. Put both your legs straight out. Put both of your hands on your toes and stretch for 15 seconds. Separate both legs as widely as you can. Bring both of your legs together. Bounce both of your legs up and down for 20 seconds. Bring both legs out straight. Point your legs straight up in the air. Try to bring your right leg back and over your head.