I know this is not a dev joke, but I laughed so hard:

Father bought a lie detector that makes "beep" whenever somebody lies around it. The son comes home this afternoon.

Father ask him:
"So, you where at school today, right ?"
Son:"Yeah"
Detector:"Beep."
Son:"OK, OK, I was in a cinema"
Detector:"Beep."
Son:"Alright, I went for a beer with my friends."
Father:"What ?! At your age I would never touch alcohol!"
Detector: "Beep."
Mother laughs:"Ha ha ha, he really is your son!"
Detector: "Beep."