As it turns out, pencil sharpeners.
A Slovak artist who goes by the name of Matus the First claims to have created the world's first advertisement "purely to promote sex and for no other reason." (In other words, it's not for a pill or a product or anything else. It simply promotes the sex act.)
He tells Britain's Mirror: "When sex becomes a routine it is necessary to change something. Therefore this is not a product commercial, because you can't buy a solution to some things. … I felt though that in some cases it might be important for people to change something in their minds, and this commercial is not only about sex but also about thinking."
We won't spoil the fun, but let's just say you might look at your desk a little differently after watching this. (Don't be confused by the Metropolitan Film Institute logo at the end; that's the production company that worked on the spot, not the advertiser.)
Fair warning: The spot might be a little unsafe for work, but it's less about the metaphor and innuendo than about the one carefully placed F-bomb. Take a look: