Joke about a lie detector robot

Here's a joke that we've found on the web that features a lie detector robot at the hands of 3 people. A father, son and mother. Who's lying and who's telling the truth? You'll find out :-) A man returns from work with a lie detector robot. His 12 year old son comes home 3 hours late from school: - Where've you been all this time? asks the father. - I was in the library doing homework! The robot homes in on the son and slaps him... The father explains: - my son, this robot is a lie detector! You'd better tell the truth... - Ok, I was at a mate's and we were watching a film: the 10 commandments. And whack! Another slap: - Ow! Actually, it was an erotic film... - I'm ashamed of you! At your age, I never lied to my parents! And whack! The robot gives the father a slap... the mother laughs and says: He's definitely your son! And whack! And mom gets a slap... via