The bump is thanks to not one, but two falls on the same spot on his head. Apparantly he has read the Toddler Manual which states "Thou shalt have a visible wound on thy head on the date of any occasion where volumes of photos will be taken."
Those of you who are observant will also take note that the picture is dated the day after his birthday. I would like to tell you that this is because of a faulty camera date, but in truth his happiness and my availability did not coordinate until the day after his birthday. Happy Birthday you poor little PK.