There are too many kinds of popular science, and the topics can be decided at will, but it should be noted that the purpose of popular science is to let the public know. So don't write too deeply, close to life, simply put, express complex science with simple meaning and let the public know.

For example, Archimedes famously said, Give me a lever and I can move the earth.

For the public, can the earth be tilted? You can say that this is an extended version of the lever principle from a physical point of view.

Or Newton's apple story, Newton discovered the law of gravity through the apple landing.

Choose your own topic. I'll set it for you in case your handwriting won't fit.