Probing Question: What is a neutrino?

Neutrinos are tiny -- really, really tiny -- particles of matter. They are so small, in fact, that they pass between, and even through, atoms without interacting at all. Neutrinos are everywhere: If you start counting now, more than 10 quintillion (that's 10 trillion billions) of them will have passed through your body by the time you finish this article. Yet only one of those 10 quintillion neutrinos will likely interact with an atom in your body. The rest will go merrily on their way.

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