Dogs and arses
The reason dogs sniff each other's arses is quite simple. Back in the day when dogs ruled the world, they would have a weekly meeting in the town hall. Before they sat down, every dog would take off its arse and hang it up, collecting it on the way out. However, the last meeting they had was interrupted when the humans took control of the world. News spread, and in the ensuing panic, the dogs were forced to pick up which ever arse was closest, and since then have been trying to find their own arse by sniffing each other's.

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