Clopen
A grid comparing closed, open and 'clopen' for sets (the whole plane is both), windows (no such thing) and minds.
Use this cartoon
Free for classrooms, worksheets, slides and other non-commercial use under CC BY-NC 4.0, with credit to Ben Orlin.
Cartoon by Ben Orlin, Math with Bad Drawings. https://cartoons.mathwithbaddrawings.com/2020-09-08-clopen/ (CC BY-NC 4.0)Transcript
[Grid: Closed / Open / Clopen]
SETS: a disk with solid boundary / a disk with dashed boundary / the whole plane
WINDOWS: a closed window / an open window / ???
MINDS: "'Clopen'? Hogwash! Balderdash! I hate it!" / "'Clopen'? Intriguing! Tell me more." / "'Clopen'? Hogwash. Tell me more."