A guy really wants to fuck his wife in the ass. But no matter what he tries she always says no. So he goes to a sex therapist for some advice. The therapist says, "There's one sure-fire solution to your problem. There's a very special herb that only grows in the Amazon. It's very hard to find, but if you go there and find it, it'll be well worth it, because it'll solve your problem."
So for ten years the guy visits the Amazon. Each year the journey is fraught with danger, including being caught by cannibals and almost dying of malaria. Finally, after years of searching, he finds the herb and brings it back to the doctor. The doc tells him: "Leave it with me, and I'll prepare it. Come back tomorrow."
The next day when he returns the doctor hands him a little bottle filled with a mysterious green liquid. The guys asks: "Okay, how do I use it, Doc?"
The doctor says, "It's easy. You put the bottle on your wife's night stand. Then you say, 'Honey, would you grab that little bottle for me?' And when she turns around and bends over to grab it, boom!, you fuck her in the ass."