He's not sure if he believes her or not. Teenage girls are, and have always been, a complete and utter mystery to him. It's been so long since he was a teenager himself, and even then the fairer sex had never really caught his interest. Her enthusiasm is baffling.
"What do you know?" She'd mentioned Tai Chi so she might have a foundation in something, or she might have a score of bad habits to unlearn. Either way, he might as well know what he's working with.
It's been years since he taught, but it's probably good for him to be teaching now. Nothing helps you see faults in your own form than trying to teach.