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Comment by drenvuk | Hacker News Reader

straws have two holes. let's say you had a y split straw. does that have one hole or two holes or three?

That's a question answered by the field of topology: genus 2.

  | |
  | +--
  | +--
  | |

Look at it end on:

    __
   //\\__
  ||  ||_
   \\//

Now shorten the shaft of the right-facing hole so that it becomes a ring-hole on the right side of the end-on hole, then rotate it also so that it's an end-on view:

   //\\//\\
  ||  ||  ||
   \\//\\//

made me go on a wikipedia dive. Thanks, I learned something.

The Vsauce video in sibling comment at 12:59 shows an animation from the two-rings to the T-shaped pipe, or even a better animation at 15:59.

12:59 https://youtu.be/egEraZP9yXQ?t=779

15:59 https://youtu.be/egEraZP9yXQ?t=959

Three. A hole is a distinct pathway from one open end to another. A regular cylindrical straw is a torus at heart, it has only one way through. A Y-straw would have three paths from its three openings.

So an X-shaped straw would have six holes?

Yep!

How many holes does a donut have? What if you keep stretching the donut making it taller?

Vsauce has a great video on this topic: https://www.youtube.com/watch?v=egEraZP9yXQ&ab_channel=Vsauc... (don't worry, completely SFW)

Replying to state that I'm wrong, mathematically.

Reply on news.ycombinator.com