Lol, part of my thumb is numb from a rat bite I got a few days ago. Reached in a cage with a momma rat to check on the babies and she NAILED me- I wasn't expecting it because this was one that was friendly before she gave birth. Stupid hormones!! She hit me right on the lower knuckle of my right thumb and GROUND her teeth. I made quite an impressive pool of blood on the floor, enough to make me lightheaded before the bleeding stopped, and did some nerve damage. I'd rather be bitten by 10 ball pythons than a pissed-off rat!

Other than that, though, a lot of friendly rats will do this 'testing' no-pressure bite, or nibble you a little- I don't worry about it much.