“But your world is made of holes,” the dragon said, stirring milky waters with a claw.
“Solid matter is mostly empty space,” he admitted, “but not like this. These are holes that are full and empty at the same time!”
“Boxes and cats,” the dragon shrugged.
In mathematics, genus (plural genera) has a few different, but closely related, meanings. The fastest, easiest and most intuitive way to introduce the concept of a genus is that it is the number of “holes” of a surface. A sphere has genus 0, and a torus has genus 1. https://en.wikipedia.org/wiki/Genus_(mathematics)