One day God came to Adam and said, "I've got some good news and some bad news."
"Well, give me the good news first."
"I've got two new organs for you. One is called a brain. It will allow you to be very intelligent, create new things, and have wonderful conversations with Eve. The other organ I have for you is called a penis. It will allow you to reproduce your new intelligent life form and populate this planet."
Adam, very excited, exclaimed, "These are great gifts you have given to me. What could possibly be bad news after such great tidings?"
"The bad news is that when I created you, I only gave you enough blood to operate one of these organs at a time."