When I was in high school, my physics teacher taught that the Moon has no magnetic field, while the Earth does.

Thanks to the magnetic field, humans can use a compass to find direction when lost in the desert. For navigation at sea, the compass is even more crucial. Without it, the Age of Discovery would never have happened.

Yet the Earth is electrically neutral overall. So why does it have a magnetic field?

I had been pondering this question since high school, turning it over in my mind whenever I had free time, much like a harmonica player taking out their instrument to play during leisure moments.

One summer vacation, I was hiking in the mountains when a sudden downpour soaked me to the skin. I took shelter under an old tree. Then thunder crashed and lightning flashed, startling me badly. I knew standing under a tree in a thunderstorm could attract lightning—charges in the clouds draw opposite charges to mountain peaks, making trees especially dangerous.

I hurried down the stone steps toward the foot of the mountain.

In that moment, a sudden insight struck me: perhaps the uneven distribution of positive and negative charges inside the Earth gives rise to its magnetic field. I was overjoyed, but how could I share this idea with others? Back then, information was scarce and there was no Internet, so I kept this precious thought locked in my heart.

Later, when I entered university, I read a newspaper report in my sophomore year.

It said a university student in Britain had discovered that the Earth’s inner and outer cores rotate at different speeds, generating electric currents—and thus Earth’s magnetic field.

This news left me both delighted and sad. I was glad that I was capable of figuring out natural laws on my own, yet saddened that I had no way to publish my findings, only to keep them hidden inside.

Years later, I joined an online science forum and posed a question: Why does the Moon have no magnetic field? Following the logic of how Earth’s magnetic field forms, the problem must lie in the Moon’s core. A quick search confirmed it—the Moon’s core and mantle rotate at nearly the same speed, with no shear motion or strong convection, so no electric currents are produced, and no magnetic field exists.

When I shared this explanation, everyone found it fascinating.