Virtually wherever I'm out and about and not wearing camo, it's just a question of time till this line starts popping in my face. Since I'm aware of provoking it on my part, I'm usually not taking offence. Once I agree, of course the actual question is inevitably always the same, so I've grown used to not even wait for it.
Depending on how exactly I'm asked (ok, and also by whom, plus how many umpteen times in the last one or two hours already), I'm not always giving a straight answer, but am rather taking the piss.
Especially at parties, when it just won't stop, and then they (mostly girls actually) start 'looking' also with their fingers to check if the lumps are really real.
And yes, they ARE, and by the way also VERY CONTAGIOUS!
As in this case I never fail to explain gleefully. (Some even believe it.)
Though my favourite standard anwer is: 'Oh, the police took me with them.'
Followed by: 'No, it's from the piercing shop. Cost me a fortune.'
And: 'As a kid, I was always bad at math, so got a calculator wetwired.'
Since (as I had to learn the hard way) 'It just grows like that' nobody believes me anyway.