a whim
Examples
My wife has a whim of iron.
Life is a whim of several billion cells to be you for a while.
Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph.
It had been a whim, and there was nothing Magnus attached more importance to than a whim.
She was afraid of all that and so much more, but what terrified her most was inside of her, an insect of unnatural intelligence who'd been living in her brain her entire life, playing with it, clicking across it, wrenching loose its cables on a whim.
A wolf was no less a wolf because a whim of chance caused him to run with the watch-dogs.