If you look closely at this picture, you can see the hula hoop fairly high in the tree. The girls are staring at it and having a lively discussion about how to get it out.
At one point, they decided to put O(5) on my niece's shoulders (she's 8), and then put E(2) on O's shoulders. It didn't work.
It was so much fun to watch them and to hear their ideas and watch them work. Eventually, their Grandpa helped them out. Fun times!