By keeping an eye on the cards they draw and discard, players can learn a lot about their tactics and possible melds. For example, it might be a sign that an opponent is trying to finish parrammy appsticular sets or sequences if they routinely remove particular kinds of cards from the discard pile. You can use this information to guide your own choices about which cards to keep or discard, upending their plans while furthering your own. Further hints about opponents' confidence levels and tactics can also be found by watching their body language and reactions while playing.
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