I got this from a different set of forums I once posted on. A friend of mine started the thread with the following concept;

There is only one restraint in this exercise, at some point in time a mirror must be found in the desert, how it got there, why the person finding it was there, everything surrounding it is left to you, the writer. The mirror could be the major part of the stoy or it could be something simply in there, and passed by without much written about it.

I guess that's the excersize.