Hahaha. Man, this comic is great, I love the expressions.
kozaim said: Why her eggplant flyed from her hands? XD
It seems you have been deceived, my good sir/madam.
To answer your question, though: He probably tightened his grip on it instinctively as a result of the rage/shock from Luka's statement. Considering the way he was holding it, doing that could easily have slipped it out of his hands.
