Magicians, carnies, and other scamsters take advantage of the fact that our brains save us a lot of time puzzling every little individual event out by instead instantly reverting to past experiences, applying them to the current circumstances, and extrapolating how to proceed from there. For instance: your brain knows, from past experience, that people do not just spontaneously change into other people.
Unfortunately, when they do, you are overwhelmingly likely to not notice at all. Stupid brain.
Embedding disabled. Give her the ol' clickeroo.