At a party this boy held up his late grandfather's pocket watch and told me that it had magic properties. He said "It tells me that you are not wearing knickers" I said "Well, that watch is not so magical, because I am" and he said "Ah, but this watch is fifteen minutes fast." A proper lady would never reveal what happened next.

uh............

hee hee hee.

Lady indeed! You're E-A-S-Y!

just to exlpain to those who did't get it: As the clock was 15 minutes fast it revealed to the boy that in 15 minutes the woman would not be wearing knickers!
It's a very funny joke!