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NEW YORK ( TheStreet) -- Investors looking to know which stocks to buy, and when, need to have the key, Jim Cramer told his "Mad Money" TV show viewers on Tuesday. But in today's markets, finding the key isn't always easy.

Cramer said that normally, with industrial production going up, he'd recommend Caterpillar ( CAT). With demand for new building on the rise, normally steel maker Nucor ( NUE) would be the go-to play. Consumer price inflation falling? Buy General Mills ( GIS). Retail sales up? Buy retail. And so on.

But in today's market, Cramer said the key isn't any of those metrics, it's the CurrencyShares Euro Trust ( FXE), a measure of the strength of the euro vs. the dollar.

Cramer recounted how as a young trader on the road, he'd only have time to ask one or two questions about how the markets were doing each day. THis trained him to find the key, the one stock that could tell him which was things were likely headed.

In the 1980s, that key was Pfizer ( PFE - Get Report), as the markets traded with the drug stocks. In the dot com boom, that stock was Yahoo! ( YHOO), and in today's market, it's the FXE.

Cramer explained that all of Europe's woes can the summed up in the strength of the euro, which the FXE measures. When the euro is strong, U.S. markets have a good day. When it's weak, our markets tank. Cramer said he wishes there was more to it, but it's just that simple.

Cramer said all investors need to have the FXE in the top left corner of their screen, above the Dow Jones Industrial Average and above the S&P 500. As goes the FXE, he said, so goes the market, no matter how good the news here at home.

Spending More

In the "Executive Decision" segment, Cramer once again sat down with Aubrey McClendon, chairman and CEO of Chesapeake Energy ( CHK), a leading producer of oil and natural gas, especially in our nation's red hot oil shale formations.

McClendon said that analysts have been concerned about the company's 2012 spending plans, but he said that Chesapeake is spending more on purpose as it moves from natural gas to more oil production, as oil is fetching four times as much as gas. McClendon reiterated that Chesapeake is not selling stock, nor taking on debt to fuel its transition, it's merely selling some of its lesser assets.

Speaking of these assets, which are worth over $23 a share, McClendon said he's perplexed as to why Chesapeake shares trade just above that number. He said the company has always been about having the best on-shore oil and gas assets and it will continue pursuing that goal even through its transition.

When asked about the natural gas bill that's been stuck in Congress, McClendon said that the hold-up has how to pay for the tax breaks on natural gas vehicles. However the latest bill tackles that problem with a 15-cent tax on natural gas itself.

"We're taxing ourselves," he said, which gives the new bill an excellent chance of passing. McClendon also said that Chesapeake doesn't need the bill to pass in order to succeed. He called a passage just "icing on the cake."

Finally, when asked about the possibility of the U.S. exporting natural gas, McClendon noted that the U.S. will have the ability to export in the next four years, but he hopes that the U.S. wakes up and uses its own fuel instead of imported oil. If that happens, no exporting will be needed, he said.

Growth Rate Key

Continuing his "Stock Supermarket" series, Cramer compared the prices of Pfizer ( PFE - Get Report) versus Celgene ( CELG - Get Report) to see which was cheaper.

On the surface, Pfizer seems cheap trading at 8.6 times earnings when compared to Celgene at 14.5 times earnings. Pfizer also sports a 4% dividend yield, a stock buyback program and a stock that's up a healthy 16% for the year. Lots to love, right? But Cramer said that P/E multiples are only half of the story, the other half being growth.

Pfizer only has a growth rate of 4%, meaning that it's PEG Ratio, multiple divided by growth, comes to 2.1. Anything over 2, said Cramer, is expensive. Additionally, Pfizer is also falling off a patent cliff, with Lipitor, its blockbuster cholesterol drug being just one of many coming off patent in the coming years.

Celgene, on the other hand, has a 25% growth rate and a PEG Ratio of just .56. Cramer said this bargain basement price is insanely cheap, given that Celgene is finding new indications for drugs like Revlimid, its blood cancer treatment, among others. When it last reported, Celgene beat estimates by seven cents a share on a 37% pop in revenues. The company also raised guidance, a healthy sign.

That's why Cramer said Pfizer is not the cheap company among these two, it's clearly Celgene.

Not Waiting for Congress

In his second "Executive Decision" segment, Cramer welcomed back David Demers, founder and CEO of Westport Innovations ( WPRT - Get Report), a leader in converting traditional engines to run on natural gas. Shares of Westport are up 168% since Cramer first got behind the stock in Jan 2010.

Demers said that Westport isn't waiting for Congress to pass a natural gas act and is instead focusing on lucrative segments that include the trash industry and oil and gas companies. He said that we'll see all kinds of applications in the future including natural gas for shipping, rail and even airlines.

Demers said that every company with a fleet of vehicles is taking about natural gas and are eager to lock in fuel prices to five and even 10 years at a time. He said that we'll see a broader range of natural gas vehicles in the next few years which will also help accelerate the adoption.

One market that's moving fast on natural gas is the trash industry. Demers said that trash trucks are contracted by cities and towns that love stable fuel prices. Trash trucks also return to base every day, making refueling infrastructure easy to accomplish. Also, with many landfills now producing their own natural gas, Demers said that trash companies are very excited to use their own fuel for their vehicles.

Cramer told viewers to stick with natural gas and with Westport Innovations.

Lightning Round

Cramer was bullish on Snap-on ( SNA - Get Report), ( AMZN - Get Report), Target ( TGT - Get Report), Dollar Tree ( DLTR - Get Report) and Costco ( COST - Get Report).

Cramer was bearish on Pitney Bowes ( PBI), Wal-Mart ( WMT), Leucadia National ( LUK), Green Mountain Coffee Roasters ( GMCR) and Marathon Petroleum ( MPC).

Kudos to Goldman

In his "No Huddle Offense" segment, Cramer called out what he called fantastic research from Goldman Sachs ( GS) that removed Research in Motion ( RIMM) from its conviction sell list.

Cramer said all too often analysts are behind the markets, recommending investors buy or sell after the move has already happened. But in this case, Goldman recommended selling the Blackberry maker last year, and since then all of their predictions have come true and the stock has fallen 71%.

So why remove Research in Motion from the sell list now? Goldman cited the company's patent portfolio as one bright spot along with its market share stabilizing. They said that the company's valuation is now consistent with its share price.

Cramer said Goldman is not advising anyone go buy Research in Motion, it's simply acknowledging that there may no longer be a good reason to sell it.

--Written by Scott Rutt in Washington, D.C.

To contact the writer of this article, click here: Scott Rutt.

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At the time of publication, Cramer was not long any stock mentioned.

Jim Cramer, host of the CNBC television program "Mad Money," is a Markets Commentator for, Inc., and CNBC, and a director and co-founder of All opinions expressed by Mr. Cramer on "Mad Money" are his own and do not reflect the opinions of or its affiliates, or CNBC, NBC UNIVERSAL or their parent company or affiliates. Mr. Cramer's opinions are based upon information he considers to be reliable, but neither, nor CNBC, nor either of their affiliates and/or subsidiaries warrant its completeness or accuracy, and it should not be relied upon as such. Mr. Cramer's statements are based on his opinions at the time statements are made, and are subject to change without notice. No part of Mr. Cramer's compensation from CNBC or is related to the specific opinions expressed by him on "Mad Money."

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