bills that are marked (or have their serial numbers listed and checked ) are not fungible.

banks generally dont check the serial numbers when you make a deposit so most cash remains fungible.

units of gold are readily interchangeable afaik.

units of #monero can be distinguished in certain circumstances. there was the MOrdinals thing for a second, those were nonfungible. if wallet data is available to The Adversary it can effect fungibility.

so like cash or gold, in certain situations certain units of monero can lose their fungibility.

on #Bitcoin everything is wide open and anyone can see tx flows. units of equal value are readily distinguished from each other.

therefore not fungible.

except in situations like Whirlpool, where the set of those utxos are *fungible with each other* but are NOT fungible with other nonWhirlpool utxos.