The challenge is to find a comic strip of the Garfield comics, where Garfield has his eyes open fully in every pannel (they are normally half closed). In "Garfield's 30th Book" there are only 3 strips where one can win the Garfield Challenge.
Tom: Hey where were you yesterday?
Bob: Oh, I was home sick.
Tom: What did you do all day?
Bob: The Garfield Challenge... I found 3, after 5 hours of reading.