Make a joke to your colleague

Basically I like to have fun and be funny in every possible way, expecially be fun through code.
As follows a joke you can make to a colleague who goes away from his desktop and leave the pc unlocked.

If your colleague was working with a browser you can leave him any kind of javascript joke, an alert, a confirm...
You can inject it through dev-console (F12) or url address bar typing for example javascript:confirm('Format your Hard Drive?').

But you can think about something more annoying...