Night Watchman

Passing an office building late one night, Paula saw a sign that
said, "Press bell for night watchman."
She did so, and after several minutes she heard the watchman
clomping down the stairs. The uniformed men proceeded
to unlock first one gate, then another, shut down the alarm system,
and finally made his way through the revolving door.

"Well," he snarled at Paula, "what do you want?"

"I just wanted to know why you can't ring it yourself."