The Doctor: Never thought I'd use this. All the times I've wondered.
Martha: What does it do?
The Doctor: Chameleon arch. Rewrites my biology. Literally changes every single cell in my body. I've set it to human. Now, the TARDIS will take care of everything. Invent a life story for me, find me a setting and integrate me. Can't do the same for you, you'll just have to improvise. I should have just enough residual awareness to let you in.
Martha: But hold on, if you're going to rewrite every single cell… isn't it going to hurt?
The Doctor: Oh yeah, it hurts.