That's a good point, and makes him flush for how intimate it feels, and yet... "Well, the truth is I can be somewhat...absent-minded, I would hate to accidentally wash it off before transferring it to my address book..." Then again, he knows how to get in contact with her for business purposes, it's not as if he'd have no way to recover the number. So he holds out his left wrist, being less likely to shake anyone's hand with it.