"I'm Telling!" was the fourth short story featured in the book Even More Tales to Give You Goosebumps.
Adam is supposed to be doing his homework, but instead he is off in a forest playing a typical game of pretend. His little sister spies on him and threatens to tell on him, since he is supposed to be inside. She repeats it many times but Adam is not at all intimidated or worried, claiming that he is pretending to do his homework. Adam then comes across an odd stone Gargoyle in the forest who suddenly begins to ooze out green liquid that turns things into stone.
Adam fills the water pistol with the liquid and sprays the tree to see that it actually does work! Missy, his sister, threatens to tell on him for that too so out of frustration Adam then sprays Missy with the water gun, turning her into stone!
On the way home, Adam decides he will use this to his advantage. Entering the Missy Stone into the Middle School Art Show and he wins first place! Second place was a drawing of a cat...
Adam wheels his stone sister back into the forest and is surprised to find the gargoyle has come to life. The gargoyle spits the green liquid at Adam's face, but Adam does not turn to stone, oddly enough. He then sprays the gargoyle which turns it back into stone. Spraying Missy again resorts in her turning back into a human.
Once again, Missy threatens Adam with more tattling about the art show and the tree and the gargoyle and about some other stuff, though it is unlikely he would get into trouble considering how unrealistic it is. But Adam, once again annoyed sprays Missy yet again and leaves her in her stone form forever.