Mary Roach, author of the new book Bonk: The Curious Coupling of Science and Sex, knows the printed page can't really communicate what it means to have sex. Turns out the penis doesn't dock to the cervix! Which is why she, and her husband, volunteered to be the first people to have their intercourse recorded in 4-D via ultrasound. Not quite sure what the fourth dimension is (time? pleasure?) but the whole thing is a little creepy, especially when it appears on Slate V, which is where we're used to seeing Dear Prudence advice letters acted out with clip art. Skip to the 2:57 for close ups of what appears to be a bladder being pushed up on.