It's true that `ls` and `find` don't take stdin, but their stdout is often piped: consider `ls | wc -l` or even something like (more complicated than) `find . -

It's true that `ls` and `find` don't take stdin, but their stdout is often piped: consider `ls | wc -l` or even something like (more complicated than) `find . -name \*.txt | xargs vi`.