Who knows? None up to that point. But if you try to use it on mom, she just gives you some additional background info. Either way, you have to use it on Kat.
It's like, a bit later, you should see, what mom and Amanda are talking about. You find them in the hallway just outside of the library. You can talk to them and get a hint to find Simon, or you can pass by them and go speak with Simon directly. In the end, you proceed to the peeping scene and the only difference is whether you'll get a bit of a story or not. Think, the same goes here.
