My friend fell off a chair today?

So today in art cl, my friend Tia (T-a) said "I'm gonna fall of my cahir on purpose. Then, I fell of my chair on purpose to be funny and she copied me. Then she started cryin' and yeah. I told our art teacher "Tia fell off a chair and now she's hurt." She said her knee hurt and that she couldn't bend it, and that she couldn't stand up but she wasn't crying anymore. I told her about the time i broke my finger. (Trying to tell her that i knew she was liying.) and she just smiled at me (She claimed that it was broken) then we got her up and she "faked a limp" i know it was fake because she went over to sharpen a pencil for me and walked to the sharpener with a sever limp and then on her way back it was magicaly gone, i think she forgot that her leg hurt because everyone was telling histarical jokes, what i want to know is why she lied to us. then i remined her and she pretended she did have a limp.... i don't get it.