Bath time revenge

I use a cup to rinse Gwendolyn's hair to keep water and soap from getting in her eyes and to make the process as quick and painless for the both of us as possible.  Tonight after doing that, Gwendolyn took the cup from me after and filled it part way, then dribbled water on her legs a few times.  When I wasn't paying too close attention to what she was doing, she filled it completely and dumped it on top of my head.  She then proceeded to laugh hysterically at her practical joke.  She was still laughing about it as I was drying her off and putting her to bed occasionally patting me on the head.