Imprisoned actor Robert Downey Jr. breaks his silence on tomorrow's Today show, giving what is being called his one and only jailhouse interview. "Prison culture is really... something that's hard to explain," he tells Jamie Gangel, Today's national correspondent. "But, it's opened my eyes to a lot. I think first, it was just kind of a shock, because... for some reason, I had expected that I was going to somewhere that was like, the treatment center next to the prison, and when I got here, I was like, it's a prison. And I was like, 'Wow.' " The 34-year-old actor was sentenced to three years in jail after he admitted during a probation hearing that he was missing scheduled drug tests. He is eligible for release on Nov. 2. Asked what the hardest part of life behind bars is, he responds: "You see razor-wire fences. You see a lot of correctional officers. Some armed, some not. All of whose job basically is to protect society from you for the time being. And that sends a message it's not a self-esteem builder."