"Don't Sit on the Gronk" is the first short story in More & More & More Tales to Give You Goosebumps.
Brad and his older sister, Kelly, are opening their Christmas presents. Brad notices a present that isn't from his parents under the tree, and he wonders where it came from. Regardless of its origin, Brad opens the box. Inside, he finds something that looks like a Koosh ball, but he sees it moving. He shows his family, and they give him permission to keep it under the condition that he take good care of it.
As it turns out, the creature came with instructions in its box. The instructions refer to the being as a Gronk. Brad, not caring much for instructions, chooses to only read a small portion of the paper. Brad invites his friend Roscoe over, and they play catch with the Gronk outside. While tossing the creature around, Brad drops it, noticing that it feels heavier than before. The creature is growing. It expands to the size of a bowling ball. As the boys continue playing with it, the ball grows to be bigger than a beach ball.
Brad takes the Gronk home, rolling it into his bedroom. The creature grows to the size of a chair, and Brad decides to sit on it, deciding it'll be comfortable. Brad soon notices a problem: the Gronk won't stop growing. Kelly enters, and she is shocked by how large the Gronk has become. She picks up the instructions from Brad's room. The kids begin running as Kelly reads.
The first rule says to not take the Gronk outside, or it will grow rapidly. The second rule says to not use it as a ball, or it will become playful. The final rule says to never sit on the Gronk because it will sit on whoever does. The giant Gronk rolls in, and it lies atop Brad. The story ends with a "SPLAT."
- This story references the Walkman and Koosh balls.
- This story shares a similar premise to the 1984 film Gremlins. Both involve a mysterious creature given at Christmastime that requires a specific set of rules to properly take care of or else it will become monstrous.