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.

What: Shares of wallboard manufacturer USG (NYSE: USG) climbed as high as 14% on Wednesday after the company posted solid preliminary results for January and February.

So what: The huge year-over-year improvement -- $5.7 million in operating profit on sales of $516.9 million versus an operating loss of $46.8 million on sales of $446.9 million in 2011 -- is forcing analysts to raise their valuation estimates yet again. In fact, the shares are busting through their 52-week high on the news and are up a whopping 150% over the past six months alone.   

Now what: Expect the short-term sales momentum to continue. First-quarter results won't be out until mid-April, but management fully expects that March's results will also reflect higher wallboard prices and expanding gross margins. Given USG's still-hefty debt load and red-hot stock price, however, Fools should proceed with a whole lot of caution.  

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