Star Wars Comment: With a small spoiler.
Ok, if you've not seen Ep III yet, and don't want to know a small spoiler, stop reading now. You've been warned.
I was reading a forum on IMDB, and someone posed the question of how Darth Vader knew that Luke and Leia were his children since at the end of the movie Vader is not sure whether his child survives, and isn't aware that there were even twins. One response made me laugh so hard I had to share.
Credit to Memnoch_TP on imdb forums.
"Vader's wife was very pregnant when she died 19 years ago. All of a sudden a 19 year old who is very strong in the force and has a distinct resembalance to him in his younger days rolls in from Vader's home planet with his old Master Obi-Wan (Who was the ONLY other person preset at the time his wife died. And to boot, the kid's last name is Skywalker.
So in answer to your question, he knew Luke was his son because his name is Darth Vader, and not Darth Retard."