Ritz does this too - she's never bitten down, just puts her mouth around a finger or hand. However when she is in full play mode, we are careful as she gets carried away and thinks we are another of her stuffed toys. We usually do the "go limp" and gentle but firm "NO", then feign disinterest so she doesn't take a more aggressive play stance. I've seen what she does to her toys - if we gave in, I'm guessing I'd have a few stitches and a little less flesh by now.