Ask HN: What is your favorite mathematical law or scientific principle? Why?

I am always absorbing more and more about the cosmos, what a joy and a blessing to feel like a kid in an adult's body.

HN is the brightest and most productive group of people I have access to currently, every day I am reminded of just how much I have to learn but also how far I have come.

Where better than here to ask this question and get quality responses from people who see the obvious value of math and science?