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I have discovered that children can begin to understand what it is all about to stand on one leg when they practice flying first. 

Helicopter pose necessitates shifting weight from one foot and then to the other.  As your child twists his body and lets his arms swing about him like a helicopter he is placing all of his weight on one foot and then as he twists to the other side the weight also shifts to the other foot.  This allows the child to place most of his weight on one foot while barely using the other foot to balance.

Once helicopter is easy for your child you can move into early stages of tree pose by asking your child to place the heel of one foot on top of his other foot while using the toes of the upper foot to help balance the pose.  Even very young children (3) can grasp this concept and often exhibit the skill.

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