I like to use extremes and the intermediate value theorem, ie if you have two extreme points, and some continuous curve connecting them, it passes through all the points in the middle. It's a good shortcut for "is it possible to balance X and Y" questions. Like is there a hexagon with 'equal' area and perimeter? It's easy to conceptualize ones on each side of the ratio, and you can smoothly deform between them so the ratio is continuous, hence there is such a hexagon.