There isn't that much about this rule that needs explaining, except to say that the best conversations of your life occur when you play along with what other people are saying.
EX: Several years ago I was at a deli eating a delicious sandwich. I was alone. A guy comes in and he orders his food. Then he sits down at a table relatively close to mine and immediately engages in a one-sided "Did you know?" conversation about -you guessed it- Bigfoot.
Of course I do not believe in bigfoot. But I said I did. I sent a hail of innocent questions toward the man, such as "How big do they get?" to which he responded, "The males can be up to ten feet tall but the females are usually human sized."
Where he received this information I do not know. UFO sites, I suspect.
Our conversation lasted until I finished my delicious sandwich. I thanked him for his time and information and left, thinking that it had been a wonderful day.