I know I should get this, I've seen it in class before...

Nice! Empty set store is both open and closed!

Maybe they should put it in a box, and then open the box?

Maybe it's Schrödinger's cafe and they won't know whether it's open or closed until they try to open the doors.

Don't go in...

probably no food in there anyways ^_^

originally i wanted (711) to be in the middle... the store 7-11 is indeed open 24/7, but viewing it as a set with one element doesn't quite fit the joke :-(

It doesn't matter if it's open or closed: once you get it, it's no longer the null set!

Unless there is no floor...

Just thought you'd like to know, I showed this to my math teacher and she got a kick out of it. Good stuff.

Nupanick now get extra credit for showing her this...

yes, but once they enter, is it a closed or open set?

Don't you know? It's called "clopen"!

Also, my Topolgy Professor tells me that "clopen" is a four-letter word, lol.