Two guys were sitting in a bar and talking together.

“What happened to your girlfriend, that really cute math student?” asks one of them.

“She no longer is my girlfriend. I caught her cheating on me.” answeres the other.

“I don’t believe,” surprised cried out the first, “that she cheated on you!”

“Well, a couple of nights ago I called her on the phone, and she told me that she was in bed wrestling with three unknowns.”