The napkin ring problem involves finding the volume of what's left of a sphere after drilling a hole through its center. It is a counterintuitive fact that the volume of the napkin ring can be written as function of its height, independent of the size of the original sphere. This is proved here using basic integral calculus to find the volume of a solid of revolution. https://en.wikipedia.org/wiki/Napkin_ring_problem How to Find the Volume of a Donut: https://www.youtube.com/watch?v=UcQKXC_JWPs learn.play.solve@gmail.com