Those ideas that suddenly appear and, just as quickly, evade your attention. Evaporating like a mist on a cool morning, its existence known but purpose lost.
Sitting in silence, replaying the thought process that got you to the point of the the spark, and being entirely unable to pin down the ghost that invaded your conscious self.
If it’s worthy of echoing it’ll resurface – most likely at a time and place that will be ENTIRELY inappropriate and out of context.
If it doesn’t, don’t worry over it. Just be happy that it once was.