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 Westport Innovations (Nasdaq: WPRT) soared 19% Tuesday after it announced an agreement with Caterpillar (NYSE: CAT) to develop natural gas engines for off-road equipment.

So what: While financial terms weren't disclosed, the deal is naturally reigniting optimism over Westport's growth prospects. Westport shares have been crushed over the past few months on concerns that the transition to natural gas-fueled vehicles isn't moving as quickly as expected, but the news suggests global manufacturers are still very motivated to take advantage of decade-low natural gas prices.

Now what: The development programs will start right away and be funded by Caterpillar, with commercial production expected to begin in about five years. "This is a significant opportunity that has the potential to transform important segments of the global off-road equipment industries," said Westport CEO David Demers. More important, with the stock still off a whopping 50% from its 52-week highs, there's plenty of room to buy into that bullishness.

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