Brad Pitt tells Oprah Winfrey that fatherhood has made him "hard as nails."

"I'm impervious to poo, snot, urine, vomit," Brad said when asked how being a dad has changed him. "You can't get me. You cannot break me down."

Brad says his children don't know how funny they really are. "Right now [Shiloh's] in this thing. First of all, she only wants to be called John. John or Peter. " he explains. "So it's a 'Peter Pan' thing."

Brad and Angelina Jolie are parents to six children: Maddox, Pax Thien, Zahara, Shiloh, and twins Knox and Vivienne.

Catch the rest of Brad's candid interview tomorrow on Oprah.