My 17 month old is slapping the backs of her legs?

I have a 15month old daughter and she mimics some of the kids my brother in law takes care. She sits down and hit the fronts of her legs. I think your baby is just trying to figure out why people do it. And to see if it's fun. When my baby does it she has fun, she laughs and gets happy. If you think your mom did something talked to her. She should tell you something. Kids get everything from tv. Just be careful that she is not watching anything bad.