There were two nuns... One of them was known as Sister Mathematical (SM), and the other one was known as Sister Logical (SL). It is getting dark and they are still far away from the convent.  

SM: Have you noticed that a man has been following us for the past thirty-eight and a half minutes? I wonder what he wants.  

SL: It's logical. He wants to rape us.  

SM: Oh, no! At this rate he will reach us in 15 minutes at the most? What can we do?  

SL: The only logical thing to do of course is to walk faster.  

SM: It's not working.  

SL: Of course it's not working. The man did the only logical thing. He started to walk faster, too.    

SM: So, what shall we do? At this rate he will reach us in one minute.

SL: The only logical thing we can do is split. You go that way and I'll go this way. He cannot follow us both.

So the man decided to follow Sister Logical. Sister Mathematical arrives at the convent and is worried about what has happened to Sister Logical. Then Sister Logical arrives.  

SM: Sister Logical! Thank God you are here! Tell me what happened!

SL: The only logical thing happened. The man couldn't follow us both, so he followed me.  

SM: Yes, yes! But what happened then?  

SL: The only logical thing happened. I started to run as fast as I could and he started to run as fast as he could.    

SM: And?  

SL: The only logical thing happened. He reached me.  

SM: Oh, dear! What did you do?  

SL: The only logical thing to do. I lifted my dress up.  

SM: Oh, Sister! What did the man do?    

SL: The only logical thing to do. He pulled down his pants.                       

SM: Oh, no! What happened then?      

SL: Isn't it logical, Sister? A nun with her dress up can run faster than man with his pants down.

And for those of you who thought it would be dirty, say two Hail Mary's!   (And I don't care if you aren't Catholic).