topology

[moloi]

show an open ball is open

[woofgang]

Do your own homework and learn some manners

[jomifl]

show me an open ball first

[squarebear]

The fact that you describe it as open means it is open.

That's like saying show me a red ball that is red.

I would recommmend you study a subject more fitting to your intelligence level.

[jomifl]

an open ball is a bit like a rod with a hole down the middle, or a square with the corners cut off.