Different levels of fungibility. 1) btc fungibility is social; it’s like currency notes we treat all btc as equal - but each btc can be traced back to when it was mined so theoretically we could say the early btc should be worth more or late btc should be worth more. its like currency notes with a number and a history of who held the note. it’s fungible only because we don’t care about these parameters socially. 2) eth fungibility is economic; it’s like notes in a bank eth gets merged in contracts and hard to trace which eth is which. it’s like a bank deposit on the notes, where you will get some notes back so practically notes are mixed up. But the bank still remembers who deposited and who withdrew. 3) zcash fungibility is technical; it’s like identical coins you don’t know anything about the ancestors of any coin in public. it’s like identical coins. no public way to know who held what coin or when it was minted. of course the holder may have a limited lineage history that they can prove. There is no one to remember the history.