If you are using paper tickets, keep your stub. After you place your ticket into the entry gate/machine it will spit it out the other side, remove it and keep it with you until you have exited the metro station. If you are stopped by a UNIFORMED transit officer within a Metro station and they ask to see your ticket you could be fined if you do not have your cancelled/processed ticket with you. After you have left the station toss the used ticket into the trash, it can be easily confused with new/unused tickets.