Community’s brick joke: 

• Remedial Chaos Theory (3x03)
• Curriculum Unavailable (3x19)

Definition of Brick Joke: Named after an old joke, which seems at first brush to be a pair of unrelated jokes. At the end of the first joke, a brick is tossed away, leaving the confused listener without a punchline. (Time passes…) At the end of the second joke, the brick returns and the listener falls on the floor laughing. -Tv Tropes

Simply put, Community did a brick joke (and extended joke over multiple episodes) about an actual brick.