Right, ok, lets try to keep up, Sam, cause I'm only saying it once! See at Christmas the Doctor got his hand cut off. This one specifically [He wriggle his right hand about.] Then my old friend Jack stole and preserved it then finally the Doctor stole it back and kept it safe till the hand was hit with regenerative properties from the Doctor, then a wonderful women Donna touched said hand and BAM! [Insert elaborate hands gestures here!] Instant biological meta-crisis. Here I am, not-quite-the-Doctor Doctor!

One heart but all the memories of the Doctor, we think the same and act the same-- sort of. I'm either 1000 years old or 1 month old, take your pick!