(Not only is this illustration overused, it's also completely bogus - at least, according to Snopes.com, the urban legends tracking website.)
But, if you plunk a frog into a pot-full of water at room temperature, then gradually heat the water until it starts boiling, the frog will just sit there contentedly, enjoying its little froggie steam-room, until it's too late.
The frog's survival instincts are geared towards detecting sudden changes, not gradual ones. Or, so the story goes.
It would be a great illustration of the perils of complacency - if it were true. Which it's not. Frogs may not be the brightest bulbs in the animal kingdom, but they're not that dumb.
It could be worse. Think of how you'd feel if you'd used it for a book title! (See Exhibit B, to the right.)
George Barna's a respected pollster and a very bright guy, but it seems he got snookered on this one. And very publicly, too.