A player holding only one card is required to call out "Uno" or risk being penalized if caught. A player who calls "Uno" risks being the target of concerted action by the other players, who may be able to use action cards to prevent that player from going out. Depending on the level and seriousness of play, some players may deliberately avoid saying "Uno", in the hope of avoiding detection and then going out on the next turn. For this reason, it is useful to conceal how many cards are in one's own hand and to keep track of how many cards every other player holds.