Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

Editor's Note: St. Joe fell an additional 12% in intraday trading on Thursday.

What: St. Joe (NYSE: JOE) stock fell 10% in intraday trading after famous investor David Einhorn said the real estate development company needed to write down assets.

So What: Einhorn gained fame betting against Lehman Brothers, and when he talks, the market listens. Einhorn is short the stock, so he definitely has a bias in his statements about the company. But he has often been right in the past.

Now What: David Einhorn is a big name in the hedge fund world and is making a big bet that St. Joe eventually will have to write down assets. Pressure from activist investors can bring actions from management, so watch St. Joe's reaction in coming days as investors start asking more questions. Do your homework before making a bet in this stock, both long or short.

Interested in more info on St. Joe? Add it to your watchlist  by clicking here.

Fool contributor Travis Hoium does not have a position in any company mentioned. Try any of our Foolish newsletter services free for 30 days. True to its name, The Motley Fool is made up of a motley assortment of writers and analysts, each with a unique perspective; sometimes we agree, sometimes we disagree, but we all believe in the power of learning from each other through our Foolish community. The Fool has a disclosure policy.