Derek punches Kumi all because she touched his plane after he told her not to touch it.
Derek: I told you not to touch it!
Kumi: You built it all wrong! Didn't you read the directions? It didn't fly for one second! It's not my fault if you made a plane that can't fly!
Derek: I told you not... to... touch it!
(Derek punches Kumi)
Derek's mom (offscreen): Derek Alan Crystal! Get over here right now!
Derek: Uh-oh! Middle name!