I heard this on Prairie Home Companion - I think Aly Bain actually told it but I don't remember.

Fergus loved books and went to a local book fair. Among the treasures he brought back was The Joy Of Sex.

He says to his wife Fiona, "Well, you see, I've bought this book, The Joy of Sex, and I've found a new position we should try."

"Well, all right, Fergus, what is it?"

"It's called the Wheelbarrow... remember when we were young, and we used to have the wheelbarrow races, where you'd get on your hands, and the person behind you held your legs up? Like that."

"All right then, Fergus, I'll try it, but on two conditions."

"What's that, Fiona?"

"First, if it hurts my back, we'll stop."

"But of course."

"Second, I don't want to go past me Mum's house."