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It can happen in a split second. A shopper leaves a purse open in a shopping cart and turns away for a moment, perhaps to browse the shelves or try a free sample, or maybe she's distracted by her phone, but that's all a pickpocket needs to reach into her purse and snag her wallet.

The distracted, rushed world we live in makes shoppers easy prey, said Bob Arno, an expert in pickpocketing who advises law enforcement on how to catch crooks red-handed.

"The thieves analyze you very well," he said. "They are like a school of fish swimming and they'll subtly watch for the weakest victim. … The one who is not paying attention."

Another classic pickpocket move, Arno said, is to work in pairs, where one person distracts the victim, while the other swoops in and does the stealing.

In surveillance video from a San Diego Whole Foods, provided to ABC affiliate KGTV by police, a man is seen getting the clerk's attention, and while the clerk's back is turned, the man's partner, standing in line behind a woman, waits for her to look away from her purse and swipes her wallet.

Arno offered a few tips to prevent people from becoming pickpocketing victims.

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1. Don't Be Distracted Just Because Someone Appears Trustworthy

Most pickpocketing instances are crimes of opportunity, Arno said.

"It's more about [people] not paying attention. … Victims make it easy for the thief," he said. "[Thieves] all say they love the person who is lackadaisical. They can practically smell them."

2. If It's Easy for You to Get Into, It's Easy for Thieves Too

Instead of leaving your purse or backpack in the cart or hanging off a chair at a restaurant, Arno suggested carrying it under your arm or keeping it in front of you.

"If I'm feeling at all threatened I have it up front," he said.

3. Keep Everything in Your Line of Sight

Never, ever leave your purse or bag in the shopping cart and walk away from it, Arno said. It will make you a prime target.