"Don't Sit on the Gronk" is a short story in More & More & More Tales to Give You Goosebumps.
NOTE: This plot synopsis was stolen from Blogger Beware's review of this story. Please rewrite this section.
It's Christmas morning and Brad has received the cutting edge gift of a Walkman portable tape-player from his parents. Brad finds another gift under the tree. It's addressed to him but there's no sender. Inside the box is a blue Kooshball. Brad is quite thrilled, as he collects Kooshballs, but when he picks the gift up, he notices that the ball is breathing. It's been a long time since I've thought about Kooshballs but I do remember something about them not being alive, so I understand Brad's puzzlement. Brad's parents see no problem with letting their son keep a mysterious breathing Christmas present and so Brad now has a pet.
Brad invites his friend Roscoe over to play with the creature. Inside the box is a paper outlining how to care for the Gronk. Brad discards the paper without reading it and goes outside to toss the Gronk around with his pal. As they lob the Gronk to and fro, they notice the creature is giggling and growing in size. Eventually it swells to the size of a basketball and the two kids shoot hoops with it. Then they take turns rolling it back and forth as the creature swells in size. Brad rolls the small boulder-sized Gronk back into his house. Once the creature is in his room, it has swelled to the size of a beanbag chair-- So Brad decides to sit on it. It's pretty comfy but the Gronk keeps growing, pushing Brad almost to the ceiling. His sister Kelly enters his room and asks him if he read the paper that came with the Gronk. As the two flee the bedroom from the ever-expanding Gronk, Kelly reads the three rules for caring for the Gronk:
Rule Number One: Don't take the Gronk outside, as this will cause it to grow. Rule Number Two: Don't bounce the Gronk, as this will cause it to become too playful.
Kelly starts to read Rule Number Three, which begins "Don't sit on the Gronk because--" when the Gronk bounces through the ceiling and lands in the living room. Brad yells as Kelly to read why he shouldn't sit on the Gronk. She finishes reading, "Because it will want to sit on you!"