What is Meant by the Reynold's Number?

This was my brother’s explanation when I asked the same question. It was so good, I felt like I needed to share it to help other people. The following are his words to me:
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Let's start with fluids. Anything that flows can be modeled as a fluid. Air, water, blood, oil, etc.
All fluids have viscosity, meaning they are sticky, and stick to surfaces.

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