The son went to his father and said, " Dad I'm feeling horny and I need 20 dollars to pay the whore for a screw."
To which his dad said, " Alright son but I have one condition, You have to prove to me that you really did have a screw. So after you are done with screwing the whore you got to put your finger up her arse and let me have a sniff of that finger".

The son agrees and goes out with the 20 dollars and after having a bang puts his little finger up her arse and on coming home lets his father smell his little finger. The next time the son is feeling horny the same scene repeats.

The third time the son asks for 50 dollars saying there is a special babe. The father agrees but on the same conditions. So after the son has his screw and sticks his little finger up the whore's arse, comes back home and lets his father smell it upon which the father suddenly exclaims, " Chauffeur get the car ready there's a new babe in town!!!"