I'm trying to remember the name of a short story I read about 25 years ago or so. I'd also like to know of any collections it appeared in.
The details I recall concern a pair of young children ( brothers perhaps) who discover a strange hole in their bedroom wall.
Somehow they discover that if they put something in it disappears and comes back changed slightly. The changes usually involve some kind of rotating/twisting jewel or bead being incorporated in some way.
They pass a few inanimate objects though, and then they decide to put the family cat through. It comes back with these jewels for eyes and acts very strangely.
I cannot remember how the story ends, it may end with one of the children going through the hole, but I cannot be sure.