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

After the stock market's worst month in nearly two years, February began even more forebodingly, as last week's jitters and a disappointing manufacturing report combined to send stocks tumbling today. All three major indexes finished down at least 2%, the second time in seven sessions that's happened. The Dow Jones Industrial Average (INDEX: ^DJI) lost 326 points, or 2.1%.

What helped spark today's sell-off was a weaker-than-expected manufacturing report from the Institute of Supply Management, which said manufacturing activity barely expanded last month, with a rating 51.3, down from 56.5 in December and worse than expectations of 56.0. It was the lowest reading in the past eight months, and new orders declined the most in 33 years. The ISM Index is one of the most widely watched manufacturing gauges, and investors were dismayed as concerns were already swirling about weak factory reports from China, emerging-market currency woes, and the Federal Reserve's stimulus taper. As many companies have said, the ISM report blamed part of the slowdown on bad weather, and carmakers also said January sales were poor as Americans stayed indoors and away from dealerships.

After hours today, Yum! Brands (NYSE: YUM) shares were heating up as the KFC-parent gained 4% after reporting fourth-quarter earnings. Per-share earnings fell from $0.72 a year ago to $0.70, though adjusted earnings came in at $0.86, beating estimates of $0.80. Many analysts had been concerned about the effects of bird flu in China, Yum!'s primary market, so investors were pleased that sales in China fell only 4% in the quarter. Yum! also stuck with its full-year forecast, further reassuring the market that the bird flu that sent a scare through China in 2013 won't affect sales, saying it expects EPS to grow 20%.

AT&T (NYSE: T) shares, meanwhile, were getting banged up, falling 4% as Ma Bell continued to feel the heat from a T-Mobile promotion, particularly aimed at luring AT&T subscribers by offering to buy out contracts. AT&T responded, saying it will end its own offer to do the same but will cut its fee on high-data plans. The move seemed to be a sign of the price wars to come in the uber-competitive telecom industry, as Verizon shares fell 3%. AT&T's decision also shows that it may lack a compelling value proposition to keep subscribers from fleeing to the smaller T-Mobile.

Don't fret on down days
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Jeremy Bowman and The Motley Fool have no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.