JetBlue will fly you from Boston to Newark, NJ, for $9. I don’t think you could catch a bus for that cheap. You may not be in either town, but you still need to care about this for one big reason.

When your product, service, experience, or stuff in general is guaranteed to change how people feel about what you do, then find ways to give it away. For free. Or close to it.

“JetBlue’s best marketing vehicle is an empty seat,” Marty St. George, their SVP of marketing, told me in Killing Giants: 10 Strategies to Topple the Goliath In Your Industry.

The variable cost of handling one more person on a flight that has an empty seat is virtually nothing. If the seat is going to be empty, it’s smart business to fill it and get what you can. But it’s brilliant business if you can take someone who isn’t “one of yours” and turn them into a raving fan. JetBlue is confident that if you fly them, you’ll want to fly them all the time.

Why would restaurant chain Denny’s give away a free breakfast – and advertise it on the Super Bowl? Two reasons. First, the variable cost of two eggs and pancakes is literally nothing. Second, they think that their breakfast, when compared with alternatives, is a lot better – and you’ll come back again. It doesn’t hurt, either, that if you also buy a cup of coffee with your free breakfast you’ll probably put them over the top on the transaction, too.

Why does free work for you? Aren’t you in business to make money?

Free works if you’ve got a business that sells stuff over and over again and what you do is fantastic. It’s word-of-mouth-worthy. If you give them a sample, they’ll come back. They’ll tell their friends.

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Key Takeaways:

What can you give away? If your stuff has a high variable cost – like your time – how can you create a lower cost version? How can you sample yourself? How many different ways can you give something of value away?

Reciprocity is a defining “decision trigger” – and it works. We often give back far more than we received when we feel the social pressure to reciprocate. Giving something unexpected, personal and highly relevant opens the doors to more business. What “gift” can you give away?

Is what you do word-of-mouth-worthy? JetBlue’s in-flight service is – and that’s why giving it away makes sense. If dogs don’t like your dog food, you need to figure out how to fix it. And giving it away isn’t the answer.

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It’s a lousy idea if your variable costs are high, if product isn’t very good, if it’s only bought once in a blue moon or if your customers have no social circles to rave to. Judge for yourself.

In JetBlue’s case, that empty seat is an excuse to start a conversation and create a raving evangelist.