Let's look at a typical novice's session with the mighty ed:

  golem$ ed
  
  ?
  help
  ?
  ?
  ?
  quit
  ?
  exit
  ?
  bye
  ?
  hello?
  ?
  eat flaming death
  ?
  ^C
  ?
  ^C
  ?
  ^D
  ?

Note the consistent user interface and error reportage. Ed is generous enough to flag errors, yet prudent enough not to overwhelm the novice with verbosity.

-- https://www.gnu.org/fun/jokes/ed-msg.en.html

:) Had not spotted the humour section on gnu.org before.

From the same site, the source code of ed:

while :;do read x;echo \?;done

-- https://www.gnu.org/fun/jokes/ed.html

Having already learned that EOT (Control+D) exited the shell, I found exiting ed and cat fairly easy.