"As a personal side-note, I've found that if you stick your fingers in the mouths of baby rats, when they get older, they are recognizably much nicer with your digits that those who did not have your fingers shoved in their mouths at an early age"
We wrestled with our youngest more as she was growing up than we did with our eldest, and she is much less likely to nip a finger through the cage than our eldest is.
I didn't know about the separating the bottom teeth thing!
I think I have to disagree with the last bite that they talk about, though, based on the nasty bite I had from our eldest rat. I stupidly put my hand in there without washing it, after handling the young rat (while they were separated before introductions). Ratty bit me frantically, squeaking and going crazy. I had one deep wound where she went in with her bottom teeth, and 6 or 7 where she was trying to grab with her front teeth to get a good grip on me. Every single one of these bite marks was defined and clearly showed the two teeth, top and bottom, side by side. :?