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The young Koffing had grown up near the power plant, enjoying a steady diet of carbon monoxide, sulfur dioxide, and other delicious gasses produced by the ever-burning Carkoal.
Until one day, the plant quit producing its tasty smoke. Koffing hadn't paid attention to the humans' planning, but they had also planted lots of Grass-types, "purifying" the air by removing all of Koffing's food. This wouldn't do. This wouldn't do at all.
So Koffing found himself on an adventure to try to find a reliable source of miasmic air to consume.
First, he followed several Grimer down to a river. They insisted the sewage that emptied into the river formed a wonderful meal, but if Koffing didn't want to get too waterlogged to move, he couldn't join them in the water to try it.
Fortunately, a small cloud of Gastly soon floated by. They seemed to have a lot in common with Koffing, so he tried to follow them to a better home. Unfortunately, he lost track of them when they floated through a solid cliff face, so he found himself lost in the wilderness, still in search of a reliable source of food. Some human campfires produced enough carbon monoxide to keep him going, but it was sparse. He was never comfortably inflated by the time the gas ran out.
Finally, Koffing found himself wandering some volcanic tunnels. It wasn't the most comfortable: the hot air kept him floating higher than he was used to, and the cave ceilings felt too close and made him long for the open sky. But the hot air also contained several flavors of sulfur oxides, more than he'd ever had the opportunity to try. The fire types liked to stay down by the lava and didn't bother him floating so high. And over time, Koffing learned to lower himself to a more familiar altitude, further away from the ceilings, only floating high away from everything when he actually wanted to be alone.
It was comfortable here. Perhaps he could call it home.