What does identical mean?

I was reading news online; Mark and Kessler were reading a book. I heard Kessler ask, "What's identical?" "It means the same. Like my butt and your face." Gawd that sounds awful. While it probably is, Mark says things like this in a very conversational tone which generally causes the kids to do a double take then say something like, "Silly daddy!" Or something along those lines. Not this time. Kess responded, "Hmm.. kinda.. except your crack is bigger and it goes the other way." Mark's antics are failing as Kessler is very analytical.