What's your secret?

Ausschwitz, 1941. A train full of jews arrives at nightfall. The leaders of the camp agree to send those jews right into the gassing chambers. After an hour the guards check if everybody in there is dead. And sure, everybody in the chambers is dead, well, except for that one guy sitting in the corner and biting his fingernails. They are astonished and send that jew to the doctor. The doc finds nothing wrong with that guy, so they send him into one of the cramped barracks.
The next day they send the guy again into the gassing chambers with another group of jews. An hour later the guards find everyone in the chambers dead except that on guy sitting in the corner and biting his fingernails. Scared they decide to inform Dr Mengele and the Führer. They arrive the next day and want to witness the guy's surving first hand. So the guy get's send in the gassing chambers with yet another group of jews. After an hour everyone is dead except that guy sitting in the corner biting his fingernails.
"Who's that guy?" the Führer asks.
"That is the bastard we were reporting", one of the guards says.
"Would you send him to me?" Dr Mengele asks. And they do. When the guy is sitting across Dr Mengele he looks nervous. Mengele examines him firmly but he cannot find anything unusual about that particular jew. So he asks: "You have been sent into the gassing chambers with almost 1,000 of your people and yet you refuse to die. Has it something to do with you biting your fingernails? What is your secret?"
The guy almost smiles as he answers: "No, it is simpler."
"Tell me your secret then", Dr Mengele orders.
"Well, you have to know, I am no jew..."