One of the most common is sex itself:

An old farmer managed to secure himself a young wife. Everyone gossiped about it, but he seemed happy.
Then one day the local doctor met Jake on the street.
"Hey doc," the farmer said, " I've been meaning to ask you about a kinda awkward problem."

"Well, I'll try. What's going on?"

"You see, doc, I ain't as young as I used to be, and I love my wife, but certain things are not as easy as they were when I was young. Sometimes I get that 'urge' when I'm out plowing, but by the time I get back up to the house, my energy and the 'urge' are gone."

"Hmmm..", says the doctor, "You know, she is young and spry. Why don't you pack a blanket and a pillow on the tractor and take your shotgun along. Then, when the urge hits, fire off a shot and have her come down to the field to you!"

"Gosh, doc, that's a great idea! Thanks."

So, about a month later, the doctor meets Jake again and asks him, "Hey Jake... that idea I gave you awhile back...umm.. how'd that work out?"

The farmer sighs sadly and shakes his head..."Well, doc, it worked just fine for a couple of weeks... then huntin' season opened, and I ain't see her since!"