After reading the question, press space. Then, just click the door when it appears to see the joke's punchline! Sorry, it's pretty corny. XD
My mom told me this joke. In Kentucky. At Mammoth Cave National Park. On a birdwatching hike. Hooray. 4/16/19: Changed the way the janitor looks! You can still see the old version, however, if you press "See inside" and look at the sprite's costumes.