The question that exposes everything
Find a nine-year-old and tell them what you do. If they say "cool", you have failed — they are being polite. If they ask a question you did not expect, you have succeeded, because a real question means a real model formed in their head.
Most of us fail this test. Not because the work is too hard. Because we explain it in the order we did it, and nobody experiences anything in the order it was discovered.
Three passes
Pass one: the sentence. One sentence, under fifteen words, with no field-specific nouns. Not "I study protein misfolding in neurodegenerative disease." Try: "I study why brain cells forget how to clean themselves."
Notice the second version has a verb you can picture. That is not dumbing down; it is choosing a better verb.
Pass two: the picture. One concrete image that survives being retold. For my work, it is a recycling plant where the sorting machine has broken, so the bins fill up and the workers cannot get to the machines. Every nine-year-old understands a bin that did not get emptied. Almost no nine-year-old understands "proteostasis".
Pass three: the stakes, in a person. Not "this could inform therapeutic strategies." Try: "My uncle forgot my name before he forgot my face. I would like the next person to keep their name longer."
That last pass is the one people skip, and it is the only one anyone remembers. Data is forgettable. A person is not.
The one test
Before you give a talk, hand your explanation to someone outside your field and ask them to explain it back to you. Do not correct them while they do it. What comes back is not their failure to understand — it is a precise map of where your explanation was never actually built.
I have done this with taxi drivers, my dentist, and a very patient group of eleven-year-olds. It has improved my papers more than any reviewer.
The objection
Someone always says: but if I simplify, I lose the nuance.
You do not lose nuance by being clear. You lose nuance by being vague. Every time I have translated my work for a nine-year-old, I have discovered a place where I did not understand it as well as I thought — and that is the real gift of this exercise. The nine-year-old is not the student. You are.