 In the scene where Eliza is practicing her "Hs", she sits down in  front of a spinning mirror attached to a flame. Every time she says  her "Hs" correctly, the flame jumps. If you look closely at the  paper she is holding in her hand when it catches fire, you will see  handwritten upon it the dialog that she and Professor Higgins have  been saying previous to this. "Of course, you cant expect her to  get it right the first time," is the first line written on the  paper.