I vaguely remember a story in which a kid was told to put some rocks in the fireplace; if a rock were moved from its place a monster could come to catch him. I Can't quite recall if it was his grandma who told him the story, but I always wondered why he didn't ignore the story and just put the rocks there in the first place?
I could have seen it on TV series or , movie but I'm not sure. It was at full color.
It sounds like the "Tales From the Darkside" episode titled "The Cutty Black Sow". From around 1988.
Here's a partial plot summary from TVTropes (The full summary on the linked page has spoilers):
A young boy's dying grandmother instructs him in a rite to ward off an evil Celtic demon that claims the souls of those who die on All Hallow's Eve. The boy obediently performs the rite, putting stones in a fire marked with the names of his family members. His Bratty Half-Pint little sister knocks the stone with his name out of the fire, which according to the myth, means that his soul will be taken by the Cutty Black Sow. The rest of the episode consists of him jumping at every sound and seeing a pair of yellow eyes through windows.
I believe the whole episode is on YouTube if you search for the title.
