A mirage is an optical illusion caused by atmospheric conditions, in particular the appearance of water in a desert caused by the refraction of light from the sky by heated air.
That’s one explanation.
It forgets entirely that this image has to appear on the retina of somebody for there to be a mirage in the first place. So the question that begs to be asked is what the hell were they doing in the desert in the first place?
The next question that pops up is whether or not there were actually people who were crawling through the desert looking for water only to see some but assume it was a mirage. Perhaps they crawled right past some perfectly good water because they saw a larger pool only a little further down the dune and that pool had some palm trees around it.
Another definition, and one that lends itself much better to storytelling, is something that appears real or possible but is not. Now this question neither begs nor pops but it ends up in my head anyway; if fool’s gold is something that looks valuable but isn’t wouldn’t a fool’s mirage be something that looks like it's too good to be true but actually is too good and true?
Somehow it still has negative implications. It might be the fact that it has fool in its name. But who else would be in a damn desert to begin with? The same fool who might argue that who’s to say that the desert isn't the mirage? One man’s desert is another man’s beach.
Except for the shiny water off in the distance that may or may not be real.
I guess it would just be nice for it to work out well for the fool once in awhile.