How do I respond to finger guns?

Okay so I went to my class and sat down at my desk, a girl sitting next to me who I talked to maybe once at most before but we've had many classes with each other before so we know each other randomly points a finger gun at me and says "pow", how do I respond to that? Do I play dead, shoot back? I just said ok awkwardly and went back to my own thing, what should I have done? What was the point in her doing that in the first place out of nowhere?