It was a hot day at Minnesota's Target Field ballpark, so my family decided to scope out the best ice cream shop in the stadium. At the checkout, before I could pay the cashier said, "Ma'am, what is your Izzy?"

"I beg your pardon," I said

"Your Izzy," she explained, "Every customer gets an extra mini-scoop of ice cream--we call it an Izzy."

"For free?" I asked incredulously.

"Yes, for free."

You would have thought my family had been asked to throw out the first pitch at the All-Star Game and be featured on ESPN. They were, to put it mildly, psyched. And it wasn't just our family, but everyone was jazzed about the free sugary stuff.

Here's the question: why would a little thing like an extra ounce of ice cream create such a wallop of joy and amazement?

Because the ice cream store, aptly know as Izzy's, offered an unexpected surprise and delight. When customers are made to feel special by something they were not expecting, the favorability for the brand increases dramatically.

In his book, Influence: The Psychology of Persuasion, Dr. Robert Cialdini demonstrated this principal. A waiter's tip increased by 3% when diners received a mint. Tips increased 14% when they got two mints. More surprising, when the waiter left one mint with the bill but returned with a second mint, tips increased by a whopping 23%.

This was a small but surprising benefit the customers were not expecting, so they, in turn, offered a bigger tip.

Why? There's a deeper psychological benefit at work: reciprocity. Dan Arierly, a behavioral economist at Duke University recently offered his thoughts in The Atlantic "If somebody does something for you," (like offering a free app, granola or ice cream at Izzy's) "you really feel a rather surprisingly strong obligation to do something back for them."

Here are a few tips to find your "Izzy" in your business:

The Importance of Relevancy-- An Izzy is particularly powerful because it is 1) something people want 2) connects to their brand 3) maps to a human insight. When Izzy's founders started out, they knew how important it was to let the customers taste the ice cream when they came into the shop. With all their flavors, Izzy's wanted to make sure the customer received "the flavor they were dreaming of." For many, (me included) deciding on one scoop is surprisingly hard. With the extra helping, we don't have to choose. The ice cream maker created the Izzy scoop as a "metaphor that physically demonstrates our approach to business--it's the little extra."

The Izzy scoop was so powerful to the ice cream maker's business, the founders trademarked it to ensure big companies don't benefit from their creative idea.

Create Social Currency--But that "little extra" does not have to be expensive to wow customers. Sometimes the most valuable ways to build good will for your product is free. Take Ryan Hoover, founder of successful company Product Hunt, a site that curates the best new products (mobile apps, websites and technology products). Hoover wanted to scale but didn't have a lot of marketing dollars. In an interview with Y Combinator partner Aaron Harris on startup school radio, Hoover remarked that when someone signed up for the service, he would send a personal email or tweet. Ask yourself, when was the last time the founder of a company sent you an email or tweet? If you answered "never" or "rarely", you can see why taking the time to send a personal note is extremely powerful.

The Dark Side of Creating Good Will – One word of caution: there's a dark side to this world of surprising and delighting customers. When you give a customer something special, whether a price discount or free gift, you have set an expectation for the customer. If you decide to pull that reward, your customer's perception of you and your brand will be less favorable than had you not had the reward in the first place. Said another way, in our Izzy example, next time I visit the store, I will be expecting a mini scoop. If the cashier tells me they have cancelled giving extra Izzy's, my disappointment will be far greater because of the expectation that has been set.

Punchline? If you are going to give a reward, make sure you are prepared to do it for the long haul; if it's for a short time, properly set expectations so you avoid customer disappointment.

What is your Izzy? Send me your ideas to @anitabnewton @inc.