a joke that goes on and on and on, with the people making the joke creating different scenarios about it.
started when some people were at the beach and a storm was coming, but there was still a green weather flag up. they saw some people in the water and decided to joke about it forever.
guy 1: "hey, look, its a green flag, but its flating in the water!"
guy 2: "its a green flag, but its ripeed and in the air!"
guy 1: "look at that green flag.... in the water!"
guy 2: "its a green flag, but its in a tornado!"
both guys: "HAHAHAHAH! ok, this is getting old."
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can be used for any kind of scenario, not just a green flag.
usually the green flag joke is only funny for a few minutes.