The biggest enemy of the bull is not the bear, Jim Cramer told his Mad Money viewers Monday, it's froth. When stocks explode higher for seemingly no reason, buyers need to be extra careful.

Exhibit A for froth has to be Beyond Meat (BYND) , a stock that came public just a month ago, and rallied from $99 to $130 a share after the company reported terrific earnings. Cramer said it's clear the IPO was priced too cheap initially, but since then many of its gains have been the result of short covering, not actual demand. When short sellers get it wrong, Cramer reminded viewers, they must buy shares in order to cover their position. That's why we see rallies like we did today. In reality, there's no way a $5 million beat in revenues should equate to $4 billion in additional market cap.

The next indication of froth are the cloud kings. After Salesforce.com (CRM - Get Report) announced it was acquiring Tableau Software (DATA - Get Report) , buyers gobbled up shares of every competing company they could find, despite the fact Salesforce is paying a huge premium for Tableau that other companies aren't likely to get.

A similar pattern emerged with United Technologies (UTX - Get Report)  deal with Raytheon (RTN - Get Report) , Cramer said. That announcement is sending investors into a frenzy looking for the next big thing.

Cramer said he doesn't believe the market is out of control, but investors need to be on the lookout for froth and avoid it whenever possible.

Executive Decision: Salesforce.com

For his "Executive Decision" segment, Cramer checked in with Mark Benioff, chairman of Salesforce.com, to learn more about their acquisition of Tableau Software for $15.6 billion.

Benioff said the customer remains at the core of everything they do at Salesforce and after the company's acquisition of Mulesoft for integration services, analytics, visualization and business intelligence were next on their customers' wish list of needs. He said every business wants to see and understand their data and Tableau is a leader in that space.

As a combined company, they can supercharge their growth, Benioff added, and Tableau believed so much in their shared vision, they opted for an all-stock deal rather than cash.

When asked about their growth, Benioff explained that he's a big believer in organic growth and innovation, but other times they simply cannot build services fast enough for customers, which is why acquisitions make make sense. Salesforce has a long history of successful acquisitions, including their marketing, commerce and integration clouds, which will all join their new analytics and visualization offerings from Tableau. 

Executive Decision: Marvell Technology

In his second "Executive Decision" segment, Cramer also sat down with Matt Murphy, president and CEO of Marvell Technology (MRVL - Get Report) , a technology company at the heart of the coming 5G wireless transition.

Murphy explained that platform leaps like 3G, 4G and now 5G are long, seven-to-10-year cycles and the 5G cycle is only beginning now. To be successful, you need expertise in processors, storage, networking and security -- all of which Marvell excels at. That's why they are well positioned to be a leader in the U.S. 5G market.

Marvell had a busy May, Murphy added, as the company made two acquisitions, divested their wifi business and announced a new strategic partnership. He said the wifi business helped pay for the acquisitions which brought them more into the connect car space. Many cars today are still using analog networking, he said, and the industry needs new high-speed digital technologies that are secure and that can scale. 

Executive Decision: Ansys

For his final "Executive Decision" segment, Cramer also sat down with Ajei Gopal, president and CEO of Ansys  (ANSS - Get Report) , the simulation software company with shares that are up 36% for the year.

Gopal said whether you're building a microchip or a rocket ship, you need the ability to design, test and validate your products and that's what Ansys allows companies to do. They allow design to get to market faster, cost less money and be more efficient.

Gopal added that it takes eight billion miles to validate an autonomous car system, for example, and that's simply impossible to do in the real world. But with simulation, companies can test thousands of cars at the same time and simulate traffic, weather and more.

Simulations and testing are no longer just at the end of the design process, Gopal said. These technologies are being taught in undergraduate engineering programs and are being used throughout the process, from prototypes to final designs. 

It Rhymes, But It's Not Good Advice

The next time you're considering taking investment advice from an old adage, think twice, Cramer urged viewers. The old saying "Sell in May and go away," is cute and it rhymes, Cramer admitted, but it has nothing to do with investing.

As President Trump's trade war heated up last month, many felt the "sell in May" edict was coming true, but that quickly reversed after the Federal Reserve declared that is was prepared to cut interest rates if the economy weakened as a result of increased tariffs. Cramer said this sudden change of heart by the Fed kicked off a huge rally, one that the "sell in May" crowd totally missed.

Selling everything is never a good strategy, Cramer explained, because it's often hard to get back in before the market turns higher. What should have been a simple move to sell into strength and buy into weakness often becomes buying into even higher prices.

"Sell in May" should be a phrase that gets retired, Cramer concluded, but until then, just ignore it and do your own homework.

Lightning Round

In the Lightning Round, Cramer was bullish on Cigna  (CI - Get Report) , Callaway Golf (ELY - Get Report) , Activision Blizzard (ATVI - Get Report) and Take-Two Interactive (TTWO - Get Report) .

Cramer was bearish on The Blackstone Group (BX - Get Report) and Roku (ROKU - Get Report) .

Ahead of E3, the 2019 Electronic Entertainment Expo, Cramer and the AAP team are focusing on Microsoft (MSFT - Get Report) and Nvidia (NVDA - Get Report) . Find out what they're telling their investment club members and get in on the conversation with a free trial subscription to Action Alerts Plus.

On Real Money, Cramer keys in on the companies and CEOs he knows best. Get more of his insights with a free trial subscription to Real Money.

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At the time of publication, Cramer's Action Alerts PLUS had a position in MSFT, NVDA, CRM.