He has to be physically pulled or slapped to focus. He also has a tendency to get distracted by "pretty" things like in "The Stolen Earth". Donna had to tell him: "Don't get all Spaceman. What does it mean?!".

I could imagine Donna insist he take her to "The Planet Of The Hats", just to make up for it.

I saw a program about a man who could remember every word and definition of a encyclopedia off by heart but couldn't button up his own shirt.