That every small, seemingly insignificant act of imagination actually has cosmic repercussions. That 3rd grade essay you wrote about the monster that used to live in your closet? Well it actually caused the birth of a little pocket universe in which your monster spawned, half formed and shadowy, given more and more substance with each reading of your essay – as long as people believed.
Convince a friend – or two, or ten – that he exists, and he gains depth and texture, and color… form a religion based on him and he gains strength and power beyond imagining.
But if you grow older, and the memory fades, and the story is lost to your past, your monster fades too, and eventually his universe shrinks and collapses and dissipates… until it’s gone forever.