Hi Ozzieace
Good question lol, to which there is no easy answer...
I say that I have a rare nerve disease called CRPS which causes me constant pain and makes my body very sensitive to things like cold, touch and wind. All those things cause me more pain. There is no cure yet and painkillers do not take the pain away, even opiates. The pain gives me headaches and affects my concentration sometimes, and can make me very down, but I work hard to appear normal.
I've found that words like 'rare' and 'disease' tend to have a little more effect on people, and even if they don't understand, they listen more to the end part. Obviously it varies

but I generally get this much info out. I then stop and don't say any more about it. If they ask more questions I'll happily answer them, but I don't volunteer too much - eyes can glaze over and you end up a social pariah!!! Some people just aren't interested in any illness that can't be cured by surgery or Nurofen lol, and you're fighting the tide there.
I'm always interested to hear how other people explain it - my version has changed several times over the two and a half years I've had it!
Bram.