When it comes to grocery stores that inspire almost fanatical devotion among shoppers, two chains come to mind—Whole Foods and Trader Joe’s. And while you can argue the merits for both, it’s clear that employees at Trader Joe’s have more or less mastered the art of bagging groceries. No ripping, toppling, or broken wine bottles here.

While you might think it's part of their employee orientation after getting a job there, it's not. According to Trader Joe’s, the skill is something their Crew Members pick up along the way. “Our Crew Members do receive some basic guidance in their training (keep cold items with cold items, balance the weight of the products in each bag, etc.),” a spokesperson for the company tells Mental Floss. “[So] most of what you see is the result of anecdotal (and effective!) tips and tricks that our Crew Members pass along to each other—as well as lots and lots of practice.”

If you don’t have a Trader Joe’s nearby or if you do and prefer to do your own bagging, we have good news: It’s possible to learn the secrets of this grocery version of Tetris. We went straight to the source: a former Trader Joe’s employee. The main takeaway? Embrace geometry.

“The best strategy I found was to ask what preferences the customer had and bag their things that way,” Christine, who worked at a Trader Joe’s in Florida, tells Mental Floss. “However, the majority of customers had no preferences, so my goal was always to make the bag as ‘square’ as possible, meaning no lumps or bumps, and consequently no items in awkward positions. This helped prevent breakage and spillage with delicate or heavy items.”

A “lumpy” bag may have produce shunted off to one side, or other items that protrude. This is where most baggers fail. “Sometimes it’s unavoidable depending on the items, [like] a bag full of produce like potatoes [or] avocados … but ‘square’ bags are easier to carry and easier to store in the car,” Christine says. “Generally they are also safer for the items inside because when the bag turns out square usually nothing is in an awkward place where it could get damaged.” 

To achieve this, it’s best to wait until all items have been scanned and then assess the inventory. Put the heaviest items at the bottom of the bag—things like canned goods—and then keep building, achieving uniformity and even weight distribution. You wouldn’t, for example, want to leave a gallon of milk on one side of the bag and put marshmallows on the other.

Aim for a lump-free bagging job.TommL/iStock via Getty Images

Once you’ve built up your base, you can put lighter items like potato chips or bread on top.

When it comes to glass, the sides aren’t where you want to go. “For wine especially this makes it more likely that the glass will break because there is a larger amount of surface area that will make impact when the bag is set down,” Christine says. “Equally dangerous is the possibility that the bottle will leak. If the bag gets wet, it will tear and everything will come tumbling out.” All liquids should be kept upright for this reason.

Almost as important as weight distribution is double-bagging. Too often, people rely on one bag to have the strength needed when two is required: “We generally always double-bagged unless the customer just had a couple of light items, or the customer requested single bags only.”

Whether to package meat separately is another customer preference. Some like bags devoted to raw foods, while others don’t care. According to the Institute of Food Technologists, you’re best off using a separate bag, as raw foods carry bacteria that may not be contained by plastic wrapping or packaging.

If you’re using reusable cloth shopping bags, make sure you send them through your washing machine (with hot water) regularly to kill any germs that might be left over from your shopping excursions.

According to Trader Joe’s, customers are welcome to bag their own groceries. When it’s busy, employees may even appreciate it. So long as you achieve a double-bagged square, everything you bought is likely to make it home intact.