Your 4yo has been coming up with various ideas to defeat gravity and fly up. Latest idea is an air box that is attached to her bum to push her up. She is desperate. She took out a piece of paper and, with help from me on spelling, wrote a letter to her science teacher. Here is how it went: For teacher's name. How to fly? How does a hot air balloon take off? She is planning to give her letter to her teacher tomorrow.