One day God came to Adam to pass on some news.
“I’ve got some good news   and some bad news,” God said.
Adam looked at God and said, “Well, give   me the good news first.”
Smiling, God explained, “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 intelligent conversations with   Eve. The other organ I have for you is called a penis. It will allow    you to reproduce your life form and populate this   planet.  Eve will be very happy that you now have this organ to give   her children.”  
Adam, very excited, exclaimed, “These are great gifts. What could possibly be bad news after such great   tidings?”
God looked upon Adam and said with great sorrow, “The bad   news is that when I created you, I only gave you enough blood to   operate one of these organs at a time.”   
Then, God came to Eve to pass on some news too. “I’ve got some good news   and some bad news,” God said.
Eve looked at God and said, “Well, give   me the good news first.”
Smiling, God explained, “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 intelligent conversations with   Adam. The other organ I have for you is called a vagina. It will allow    you to reproduce your now intelligent life form and populate this   planet.  Adam will be very happy that you now have this organ to give   him children.”
Eve, very excited, exclaimed, “These are great gifts   you have given to me. What could possibly be bad news after such great   tidings?” 
God looked upon Eve and said with great sorrow, “The bad news is that when I created you, I only gave you enough blood to operate one of these organs at a time. While you’re bleeding through one of them every 28 days, the other will remain useless.”