A long while back, I read a scifi book (really a kids scifi book) about a kid that finds a red pebble which cancels gravity for anything it's in contact with (except kids, obviously), causing those objects to be pulled strongly up into the sky. Why? Because they were attracted by the moon's gravity. They even got it to spin round and suggested using that for free energy. Can't for the life of me remember the title or author, but I remember it really grated against my sensibilities, and I never read anything by the same author again.