There are two nuns. One is Sister Logical (SL), and the other is Sister Mathematical (SM). It is getting dark and they are walking home, but 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 minute? 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 do 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 only logical thing for him to do, is 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 for us to do is to split. You go that way and I'll go this way. He cannot follow us both.
They split and the man decides to follow Sister Logical. Sister Mathematical arrives at the convent and is worried about what has happened to Sister Logical. Then Sister Logical finally 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! And then what happened?
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.
SL: The only logical thing happened. He reached me.
SM: What did you do?
SL: The only logical thing happened. I lifted up my dress.
SM: Oh, Sister! What did the man do?
SL: The only logical thing happened. 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 a man with his pants down!
(And for those of you who tought it would be dirty, pray for forgiveness.)