Bill Gates Says This Is the ‘Ultimate Test of Whether You Know a Topic'—He Even Tried It on His Kids

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Bill Gates believes there's only one way to know if you truly understand a subject: Can you explain it to a child?

The billionaire Microsoft co-founder's lifelong love of math was put to the test when he tried explaining mathematical concepts to his then-school-aged children — and, it didn't always go smoothly, Gates said in a recent keynote talk at the ASU+GSV Summit in San Diego.

"Learning with kids and seeing what's confusing to them, what's easy for them, is kind of the ultimate test of whether you know a topic — whether you can explain it," Gates said.

There's a reason teachers often ask students to give presentations on a subject to demonstrate that they understand it: People are more likely to remember and understand concepts they've learned after explaining it to someone else, or even to themselves, psychology studies show.

That means you can test your own knowledge in a subject simply by trying to teach it to a friend, to see both how much information you've retained and how well you actually understand its concepts.

If they successfully learn from you — which, in fairness, may somewhat depend on your friend — it's a good sign that you're on the right track.

Gates, who studied math and computer science at Harvard University before dropping out to start Microsoft, noted that while he personally excelled at math as a high school student, he had trouble passing that enthusiasm to his three kids.

"I love doing math, [but] my younger daughter would always say, 'You made it so complicated,' and 'You didn't need to explain this to me to get the answer. Why did you go through that whole general description of angle, side, angle? You could have been done 10 minutes ago,'" Gates said. "She just wanted the answer."

Today, Gates' three children are all in their 20's. While they may not have enjoyed the math lessons, Gates still said his teaching sessions were a "great" experience — at least, for him — because they reinforced his own love of math.

"One of my favorite things was teaching calculus to the kids .... It's a few very difficult concepts, like area under the curve and rate of change. You have to explain why that's so important, and why they have those funny symbols," Gates said, adding: "It's crazy. It's probably the thing I enjoy the most."

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