The Magic 8 Ball is a toy Greg used to make decisions. It was originally owned by Rodrick, who got it as a present the year before but seemingly forgot about it. Greg used it for many decisions, such as if he should go outside and play, what after school activity he should choose and more. The Magic 8 Ball was very useful for Greg, and it assisted him for a school test - however it broke in Mrs. Meritt's class, and he is caught for allegedly "cheating". Greg decides to toss it over his Gramma's fence. He later finds it next to a plastic Easter egg containing Meemaw's diamond wedding ring.
The Magic 8 Ball is actually made by Mattel Inc., the company that made Monster High, Barbie and Hot Wheels. This is the second Mattel product to appear in the series, after the Barbie Dream House in Diary of a Wimpy Kid.