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.

Another day of mixed economic data left the iconic S&P 500 (SNPINDEX:^GSPC) vacillating within a tight range, and gave investors another day of pause at the beginning of the new year following a nearly 30% gain in 2013.

On one hand, U.S. auto sales hit a six-year high with December's sales figures leading to a grand total of 15.6 million units being sold in 2013. However, most economists had predicted the auto sector would pull in about 16 million units this year. This shortfall demonstrated that consumers were choosing to scale back their car purchases around the holiday season. Domestically, Ford (NYSE:F) managed to pull out a 2% unit sale gain with its F-Series pickup continuing its now 32-year streak of being America's best-selling vehicle, while General Motors delivered a 6% decline in unit sales. Both figures missed Wall Street's expectations.

Also adding to the markets' uncertainties was a sizable decline in oil inventories of 7.01 million barrels, marking the fifth straight week of declines. Although oil prices have declined notably during the first two sessions of 2014 on weakness in China's manufacturing data, this precipitous decline in inventories could be worrisome for the future price of oil – and let's recall that higher oil prices can negatively impact consumers in their pocketbook.

By day's end, the S&P 500 ended fractionally lower by 0.61 points (-0.03%) for its second straight session of losses, to finish at 1,831.37. Despite the broad index moving fractionally lower, though, three stocks exploded to the upside.

Leading all companies to the upside today was malware protection service provider FireEye (NASDAQ:FEYE), which advanced 38.6% after announcing the acquisition of endpoint Internet security and response company Mandiant for approximately $1 billion. The deal makes sense for both parties, giving Mandiant a reasonable premium, and allowing FireEye to attack both ends of Internet security including the proactive and reactive components. To add fuel to the fire (pun fully intended), FireEye also announced that its fourth-quarter and full-year revenue and billings topped its previously announced guidance. In 2014, for example, FireEye is now anticipating total billings will be $540 million to $560 million, up from a prior forecasted range of $350 million to $370 million. While I wouldn't advocate chasing FireEye higher at the moment, a moderate pullback could represent an intriguing buying opportunity.

Continuing with the theme of alternative energy stocks soaring, FuelCell Energy (NASDAQ:FCEL), a designer and servicer of fuel cell power plants, gained 17.4% despite a lack of company-specific news. Last week, however, FuelCell Energy did issue a press release that it had completed a 14.9 MW fuel cell park for project owner Dominion Resources, which comes with a 15-year contractual energy purchase agreement. Like many of the green energy stocks we reviewed yesterday, FuelCell Energy is certainly making strides to prove its long-term viability, but it'll need to make a serious effort to reduce its losses and generate recurring cash flow before I'm sold that it has what it takes to survive and thrive.

Finally, fuel cell systems developer Plug Power (NASDAQ:PLUG) snuck its way onto the top performers list for a second day in a row with a gain of 12%. While no news was released today that would directly be responsible for moving the share price, Plug Power is likely pushing higher as a continuation move from yesterday when it announced that it had met its order targets for the fourth quarter, which would net it about $32 million in revenue. As I pointed out yesterday, Plug's big-name customers are definitely a plus, but it'll need to make serious strides in reducing its bottom-line losses in 2014 if it hopes to maintain its hefty two-day gains.

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Fool contributor Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong, track every pick he makes under the screen name TrackUltraLong, and check him out on Twitter, where he goes by the handle @TMFUltraLong.

The Motley Fool owns shares of, and recommends Ford. It also recommends Dominion Resources and General Motors. Try any of our Foolish newsletter services free for 30 days. We Fools may not 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.