You shouldn't peak at your friend's private parts.
Jack is at a slumber party with some of his friends, and his friend Dale goes into the bathroom to change his clothes.
Jack wants to play a prank on his friend using silly string.
Jack plans with his other friends to wait until Dale gets back before they jump him and shoot him with the silly string.
Jack runs into the bathroom and sees his friend naked before shooting him with silly string.
Dale is surprised but a good sport about the joke, and they have a good time at the party.
Jack's friend feels uncomfortable being seen naked and it makes the rest of the party awkward.