Afterwards Tom and Eric weren't exactly sure at which point during their discussion the elephant had entered the room
"Elephant in the room" (also "elephant in the sitting room", "elephant in the living room", "elephant in the parlor", "elephant in the corner", "elephant on the dinner table", "elephant in the kitchen", and "elephant on the coffee table") is an English idiom for an obvious truth that is being ignored or goes unaddressed. The idiomatic expression also applies to an obvious problem no one wants to discuss.
It is based on the idea that an elephant in a room would be impossible to overlook; thus, people in the room who pretend the elephant is not there might be concerning themselves with relatively small and even irrelevant matters, compared to the looming big one.
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Thanks to the following folks for posting their work as creative commons so that I can create these pictures.