“Can I pick your brain?”

It’s a moment many of us dread. A stranger asks for a phone call—or worse yet, a meeting. You already have way too much on your plate. Your brain is not just sitting there passively, waiting to be picked. Plus, the mental image of a brain being picked is just gross.

Most people answer this question by asking, “What will I get?” They run a cost-benefit analysis, and base the decision on expected utility. And that’s a terrible mistake, because it’s impossible to predict the utility of a meeting.

In 1994, a professor named Bill Hamilton was asked to meet with a young entrepreneur who was starting a biotech venture, and consider joining the board. After one look at the business plan, “I quickly decided that I had no interest in a venture with no money and no management,” Bill says—there were “just too many other demands on my time.”

But then Bill took a leap of faith. Despite having no desire to get involved, “I offered to meet with him to see how I could help.”

Bill asked the entrepreneur a series of questions, and ten days later he had a set of unusually thoughtful, detailed answers in his hands. After several more meetings over the next few months, Bill was impressed enough with the entrepreneur and his ideas that he changed his mind, and joined the board as the first independent director.

Seven years later, just before an initial public offering, they sold Avid Radiopharmaceuticals to Eli Lilly for $320 million. “This was a terrific exit for everyone associated with the company, including investors, management and directors,” Bill says. “I was only trying to help, but I ended gaining a lot—both experiential and financial.”

Call it the Law of Unexpected Utility. By definition, you cannot know when you meet someone today where that person will land tomorrow. If you make every decision by asking “What will I get in return?” you’ll miss out on these moments of serendipity.

Of course, we all need to set boundaries. Serial entrepreneur Adam Rifkin protects his time by doing 5-minute favors: micro-loans of his knowledge, skills, and connections that cost him little but can add a lot of benefit to others. Venture capitalist Brad Feld holds Random Day: every other month, he opens up a day in his calendar to meet with anyone for 20 minutes. That allows him to take about 80 meetings a year with strangers, which only costs six days of his time.

The point is not to get something. It’s to give something. Often, giving is a better path to success than taking. Even when it doesn't pay off directly, givers lead more meaningful lives than takers.

So next time you get a brain-picking request, you might consider taking the conversation. And if you’re the one looking for guidance, there’s a better way to show respect than asking to pick someone’s brain. “Can I seek your advice? I’d value your insight because…”

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Adam Grant is the youngest tenured professor at Wharton and the acclaimed author of Give and Take. His second book, Originals: How Non-Conformists Move The World, will be released in February 2016. Learn more about Adam at www.adamgrant.net.