Why do we say.....

[Roughquest]

When we are familiar with a place, why do we utter the phrase "Oh I know it like the back of my hand".......meaning what exaclty......

[Quizmonster]

The idea is that we are all automatically familiar with the backs of our hands, given that we see them so frequently throughout every day.
Personally, I've never believed it. I'm certain that, if I was faced with ten photographs of various hand-backs including my own, I'd be unable to pick mine out. I suspect the same would be true of most people, in the absence of anything particularly distinctive.

[Thunderbird+]

Quizmonster,
I recon you must wear glove's a lot, if you dont think you can reconise your own hand!
Now I'me wondering if it's ever been put to the test ?