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popped
Every buy of $POP pumps air in. Every buy might be the one that pops it. Nobody knows which.
Any buy of $POP adds air, as much as the ETH it spent. Sells let half that much out.
The fuller it gets, the more likely the next buy pops it. Whether a buy pops it comes from the hash of the block it landed in, so nobody can know in advance, not even the buyer.
The buy that pops it goes on the wall, with how big the balloon got. Then a new one starts.
Anyone can check a pop: for each buy, take keccak256 of its block hash and log index, divide by 2²⁵⁶, and compare with the pop chance shown for that buy. The formula is in this page's source. The chain's sequencer orders blocks, so in theory it could lean on a block hash.