When someone is asleep with his or her mouth open, and you place your cock in his or her mouth and let them wake up to a mouthful of cock. Surprise! It is also possible to ejaculate in the open mouth and allow them to wake up with a foul taste. Ideally, both should be done to achieve full effect of the surprise. Not only will they realize the foul taste in their mouth is semen, but they also see that it is from your cock.
"Julian fell asleep with his mouth open again in his dorm room. I set his alarm clock and waited for an hour, but it was worth it to give him a morning mouthful. Man was he mad, especially since he's straight."