I cannot enter a title, because it is a title

The most absurd UNIX thing I've encountered:

$ rmdir zc_install/
rmdir: zc_install/: Is a directory

I don't know how to explain this to non-UNIX types. It'd be like in Windows, if you went to open a file, and instead of opening it you got an error saying, "Error: Cannot open this file, because it is a file."