If you can't explain it to a six-year-old ...

"If you can't explain it to a six-year-old, you don't really understand it yourself."

A line often credited to Richard Feynman, the Nobel-laureate physicist.

I learned the same lesson, harder, from Professor Nicholas Bloembergen, my PhD thesis advisor at Harvard. (He won the Nobel Prize the year I completed my PhD.) He told me — before we got anywhere near the difficult calculations — to first use simple maths and rough estimates to check whether we were even in the right ballpark.

It forces you to find the essence of the thing. Once you have the essence, you can explain it to a six-year-old.

At an education conference, I heard Howard Gardner — the Harvard psychologist behind the Theory of Multiple Intelligences — argue for what he calls pedagogical intelligence: the ability to teach complex ideas simply. He has not formally added it to his eight intelligences, but he believes it deserves a place there.

I think pedagogical intelligence is the most important professional skill of the next 20 years.

Here is why. AI is collapsing the cost of complex output — code, contracts, reports, diagnoses. What it does not collapse is the ability to take something complicated and make it land for a particular person. That requires understanding both the subject and the human in front of you.

When we hire preschool teachers, this is one of the things we screen for. We do not need teachers who can recite the curriculum. We need teachers who can explain to a six-year-old that DNA is the blueprint for building life — just like the instruction manual in the Lego box that tells you how to build the dinosaur. A teacher who can turn that idea into a tomato-DNA extraction experiment that a six-year-old will remember for the rest of her life.

If you are choosing a preschool, watch how the teachers explain things to your child. Not how the marketing brochure describes the curriculum.

When your child comes home from school, what is the most surprising thing she has tried to explain back to you?