http://monerotoruzizulg5ttgat2emf4d6fbmiea25detrmmy7erypseyteyd.onion/el/resources/moneropedia/fungibility.html
With Bitcoin, any BTC can be tracked by anyone back to its creation coinbase transaction . Therefore, if a coin has been used for an illegal purpose in the past, this history will be contained in the blockchain in perpetuity. This lack of fungibility means that certain businesses will be obligated to avoid accepting BTC that have been previously used for purposes which are illegal, or simply run afoul of their Terms of Service.