When I was in high school and saw the following equation, my mind was blown!
Why is Pi here? Isn’t it supposed to show up only in circle related problem? But the left-hand has nothing to do with circle. And it’s Pi squared!
Even today I still do not have an intuitive understanding why the above equation is true, until I watched a visual explanation from 3blue1brown. This video provide an elegant and visual proof of the equation. Go ahead and watch it: