Many of the questions I get asked these days begin with, “Is it true that ...?” and can be devilishly hard to answer.

Usually, the questioner has seen or heard something, often through social media, about a loathsome substance that is set to destroy their life or about one that promises to improve their health “naturally.” The answer to the question is almost always “no, it isn’t true, but ... The reason for the qualifier is that science isn’t white or black, it is best seen as several shades of gray.

Generally, I first try to explain what we really mean when, in a scientific context, we claim to “know” something. It almost never means that we are absolutely certain. I say “almost,” because there are some certainties: The Earth is not flat. It orbits the sun. Carbon-13 — but not carbon-12 — is radioactive. A hammer and a feather will fall with the same speed in a vacuum. A dog cannot mate with a cat to produce progeny.

However, when it comes to what science says about matters of health and longevity, the answer to “is it true that?” is rarely an unqualified “yes” or “no.”

When asked whether it is true that a glass of orange juice contains roughly the same amount of sugar as a glass of Coca-Cola, the answer is a definitive “yes,” because this can be proven with chemical analysis. The answer to whether that glass of Coke contains cocaine is for all practical purposes a “no,” but nuanced. While the coca leaves used to prepare Coke’s famous formula 7X have had their cocaine content removed, it is possible that inconsequential traces are left behind. Is true that rat poison is being added to tap water? Yes, but obviously with a qualifier. Sodium fluoride is added to tap water to........

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