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NEW YORK (TheStreet) -- When the markets open up big, investors should be selling, not buying, Jim Cramer told his Mad Money viewers Thursday. Cramer said he hates up openings, especially when all the signs are still flashing red. Wait until the afternoon when the markets fall, then start your buying.

Nothing's changed since the market's big selloff earlier this week. There is still no resolution to the Greek crisis, although one continues to be promised "any day now." China's economy is still sinking and sinking fast. And here in the U.S., any and all positive news is still being met by the Federal Reserve talking about interest rate hikes.

So why then would anyone be pay up for stocks first thing in the morning when everything is still bearish? All of these negative are factors beyond a company's control, and that's a bad backdrop in which to be paying a premium.

Instead, Cramer suggested waiting for the markets to fall, as they did this afternoon, and then buy the stocks of well-run companies like Action Alerts PLUS holding Walgreens Boots Alliance (WBA) - Get Report, which was up 4.2% today, or Pepsico (PEP) - Get Report, which continues to innovate, or T-Mobile (TMUS) - Get Report, which continues to bash its competitors while slowly stealing all of their customers.

Bulletproof Stocks

With the broader markets continuing to drift lower, investors need stocks that are bulletproof, which is why Cramer offered up 15 stocks he said are safe havens since all of them are fresh off the 52-week high list.

Walt Disney (DIS) - Get Report has terrific franchises and theme parks, Cramer said, and is worth buying on any weakness. The same applies to Nike (NKE) - Get Report, the footwear powerhouse that continues to dominate.

Americans are keeping their cars longer, Cramer said, and that's means O'Reilly Automotive (ORLY) - Get Report, along with Advanced Auto Parts (AAP) - Get Report and Snap-on Tools (SNA) - Get Report are all buys.

Housing continues to be a strong trend, which is why carpet and flooring makerMohawk (MHK) - Get Report made Cramer's list, along with Stanley Black & Decker (SWK) - Get Report.

Others that made Cramer's hot list were oil refiner Marathon Petroleum (MPC) - Get Report, Comcast (CMCSA) - Get Report, the definition of consistency, along with Electronic Arts (EA) - Get Report and Eli Lilly (LLY) - Get Report.

In the food space, Cramer recommended General Mills (GIS) - Get Report, the new Kraft Heinz (KHC) - Get Reportwith its 3% yield and Mondelez (MDLZ) - Get Report.

Finally, Cramer gave the nod to Ulta Salon (ULTA) - Get Report, which continues to power higher as its turnaround gains steam.

What Cramer Learned From Alcoa

What does good investment homework look like? Cramer told viewers what he parsed from reading Alcoa's (AA) - Get Reportearnings yesterday as well as listening to the company's conference call and talking to its CEO on last night's Mad Money.

Cramer said his overall takeaway from Alcoa was that the U.S. is doing a lot better than people think. Furthermore, since Alcoa breaks its business down by end market, there were a lot of insights to be gleaned.

Alcoa cited strong growth in aerospace, which led Cramer to think of Boeing (BA) - Get Report, Honeywell (HON) - Get Report, another AAP holding, and BE Aerospace (BEAV) .

Alcoa also said that natural gas turbines were up 19% in 2015, which confirmed Cramer's thesis that coal is in decline in the U.S. while natural gas will only get hotter.

Then there's the strength in packaging, mainly new aluminum beer cans, which led Cramer to think that Constellation Brands (STZ) - Get Report might be worth a second look.

Finally, in the auto market, light trucks remain strong. However, given their global exposure, Cramer couldn't recommend any of the truck makers.

All of this is what homework looks like and how investment theses get started, confirmed or disproven, Cramer concluded.

Merger Mania

The markets may be in a standstill waiting on resolutions in Greece and clarity in China, but that's not stopping savvy CEOs from making smart acquisitions to bring out value. Cramer highlighted just a few of these recent great deals.

First up was ACE (ACE) buying Chubb (CB) - Get Report, a combination that creates the number one global property-casualty insurer, all while saving an estimated $650 million a year. Smart.

Then there's Centene (CNC) - Get Report paying a 21% premium for Health Net (HNT) , a deal which also yields huge cost savings for the new entity. Cramer said he's a buyer of Centene.

Next is Horizon Pharmaceuticals (HZNP) - Get Report making a hostile bid for Depomed (DEPO) , which would be just the latest of Horizon's smart acquisitions.

Finally, there's the big Aetna (AET) and Humana (HUM) - Get Report tie-up. In an industry where scale matters, this new company will have it in spades. And let's not forget the rumors about Cigna (CI) - Get Report and Anthem (ANTM) - Get Report. That deal may be back on.

Lightning Round

In the Lightning Round, Cramer was bullish on Boeing (BA) - Get Report, Dillard's (DDS) - Get Report and Waste Management (WM) - Get Report.

Cramer was bearish on Mastercard (MA) - Get Report, Progressive Waste Solutions (BIN) , Novavax (NVAX) - Get Report, Timken (TKR) - Get Report and Rio Tinto (RIO) - Get Report.

Off the Tape

In his "Off the Tape" segment, Cramer sat down with Philip Krim, CEO of the privately held Casper, a company disrupting the mattress industry by allowing customers to order the company's high-tech memory foam mattress online, then have it delivered to their door via UPS (UPS) - Get Report, just like all your other online purchases.

Krim said Capser has one of the most comfortable mattresses in the world thanks to a unique combination of materials that has never been tried before. The company also has special machines that can compress the memory foam mattress into a box the size of a mini fridge, which allows it to be delivered by traditional channels and eliminates the hassles of getting it into your bedroom.

Casper just completed its second round of funding, snapping up $55 million in capital to continue the company's growth.

To watch replays of Cramer's video segments, visit the Mad Money page on CNBC.

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At the time of publication, Cramer's Action Alerts PLUS had a position in HON, MA and WBA.