Riemann hypothesis
Sometimes when you put an input into the Riemann zeta function, you get the number zero back. When this happens, you call that input a root of the Riemann zeta function. You call the input a "root" when it gives you zero. Lots of roots have been found. But some roots are easier to find than others. We call the roots "trivial" or "non-trivial". We call a root "trivial" if it is easy to find. But we call a root "non-trivial" if its hard to find. The trivial roots are numbers called "negative even integers". The reason we think they are easy is because they are easy to find. There are neat rules that say what the trivial roots are. We know what the trivial roots are because of the equation that Bernhard Riemann gave. That equation was called "Riemann's functional equation".