Ben Stiller thinks sex jokes are funny because they are "forbidden".
The actor is promoting his new movie 'The Watch', in which he plays a man called Evan who is unable to have a child with his wife.
For the movie, Vince Vaughn, Ben Stiller, Jonah Hill and Richard Ayoade come together as a neighborhood watch group who are desperately seeking an escape from the daily drab of their suburban lives. Inadvertently, the crew becomes responsible for saving the world from an alien invasion.
The film features a lot of dirty jokes, some of which centre on Evan's personal problem.
"I think sex is just something that people can't talk about in public usually. There is suppression of that or a repression that makes anything you can't talk about vaguely forbidden and the humour can come out of that," he told British newspaper The Independent.
Ben thinks people would be surprised to know what he is like in real life. While he often plays characters who are orderly and always think ahead, he isn't that way at all.
"I'm really not that outgoing or, for that matter, particularly organised," he laughed.
Check out our 'least sexy celebrity kisses'... now that is funny.