I was just reminded of my favorite thing in all of #mathematics, which is the final sentence of Pierre de Fermat's "Last Theorem." It was written in the margin of a book, and he made a claim about the sums of powers, but he didn't provide a mathematical equation to prove it. Instead, he wrote this absolute gem below the claim:

"For this, I have found a truly wonderful proof, but the margin is too small to contain it."