Thank you for your reply.
I am not trying to find a way to "clean" my bitcoins, I don't have dirty ones (unless those I received are). But anyway, if there ever is a problem, I can prove I paid them with my money anyway....
No, my question is really about learning. I can't visualize how could it be possible to trace each satoshi is they don't have a kind of serial number ? The only way I can see it could be done if there are no serial number is by painfully digging the blockchain and trying to match the inputs and outputs and wallet addresses and time-stamps, and "build" the tree of connections. Which could take you years to compile... no ?
How? The block chain is a public ledger. Every transaction is recorded and displayed. No serial number is needed.
A Bitcoin is created as a reward to the miner. At that point each and every transaction can be traced. 0.01 BTC was sent on this date/time to that wallet. Which inturn sent a portion to another wallet.
It would be a difficult chore for a human to trace this with pen and paper. But it's not difficult for computers.
As a matter of fact miners ensure there is no double spending by tracing the transaction history.