Cat Flap for a Cube
A square pet door lets a cube through, and also a very long square prism, which explains it came in 'one cross-section at a time.'
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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/2022-01-01-cat-flap-for-a-cube/ (CC BY-NC 4.0)Transcript
A (pointing to a small square flap in the door): "There, little cube! Now you can come and go as you please!"
[Later: a long square prism lies in the room] A: "How did YOU get in here?" Prism: "One cross-section at a time."