One sunny day in the Garden Of Eden Adam was sitting with Eve when he noticed that a lot of the animals in the garden would often appear from out of a thicket of dense trees with big grins right across their faces.  He was puzzled as to the reason why, so he thought he'd go and ask the Lord for an explanation.

Adam explained to the Lord what he'd seen, who replied "It's called procreation, Adam.  Why don't you take Eve into the forest with you and find out about it for yourself?"

So Adam decided to take the Lord's advice.  However, less than twenty minutes later, the Lord was dismayed to see that Adam was back again, looking very puzzled indeed.

"Yes Adam?" said the Lord.  "What is it this time?"

"Lord" said Adam "What's a headache?"