True story.

In college I had a crush on a very beautiful woman who, at that time,
was having difficulty with her marriage. A mutual friend told this
joke to her to illustrate that sex is all in the mind.

Q: What's a four letter word for intercourse, ending in "k"?

My beautiful friend, petite and fair, instantly turned a very becoming
shade of crimson, but said nothing.

"Why are you blushing?" says our mutual friend. "The answer is
'talk', obviously!"

My beautiful friend was delighted and said she must try the joke on
her husband. Next day she tells this sad story.

"I asked my husband the question and he thought for a moment before
saying: 'Well, speak. But that's five.'"

Looking most dejected she continued "and that's the whole trouble with
our marriage."