[jennyp]

It's not a fancy trick; it's just a ramification of the definitions for square and circle. They don't exclude the dengenerate case.

We don't allow degeneracy on this board!

[longshadow]

We don't allow degeneracy on this board!

This brings to mind the image of an aging man dressed in long raincoat, hanging out in the shadows behind the Math Department whispering to passers-by:

"Pssst! Wanna buy some degenerate geometric figures?"