One game of Gin rummy followed from the deal to the finish, move by move, with the reason for each choice.
Ada deals ten cards each and turns up 7♦ to start the discard pile. As the player who did not deal, you go first. Your hand: 3♣ 4♣ 5♣, 7♥ 7♠, 9♦, J♠ Q♠, K♥ and 2♦.
You already have one meld, the clubs run 3-4-5. Everything else is deadwood, counted at face value with court cards as 10: that is 7, 7, 9, 10, 10, 10 and 2, for 55.
7♦ on the pile makes a set with your two sevens, so you take it rather than drawing from the stock. Now you must discard, and K♥ is the obvious card: it is worth 10 and has nothing near it.
Your melds are 3♣ 4♣ 5♣ and 7♥ 7♠ 7♦. Your deadwood is 9♦, J♠, Q♠ and 2♦, which comes to 31.
Ada draws from the stock and throws 8♣. It does not help you, so you draw from the stock yourself: A♥. You throw 9♦, the highest card that is not close to a meld, and your deadwood is J♠, Q♠, 2♦ and A♥, for 23.
Ada draws again and throws 10♠. That joins your J♠ Q♠ into a run, so you take it. Your hand is now 3♣ 4♣ 5♣, 7♥ 7♠ 7♦, 10♠ J♠ Q♠, plus 2♦ and A♥.
You have eleven cards and must discard one. Throwing 2♦ leaves A♥ as your only deadwood: 1 point, well under the 10 you need to knock. You could hope to draw a card that makes gin, but every turn you wait is a turn Ada could knock or get low enough to undercut you.
So you knock, discarding 2♦, and lay your hand out: three melds and A♥. Ada lays out her hand: 8♥ 9♥ 10♥, K♣ K♦ K♠, and 5♦, 6♠, 7♣ and J♥ loose. She may now lay off loose cards on your melds, and 7♣ fits on your three sevens. Her deadwood is 5, 6 and 10: 21.
You knocked with 1 and Ada has 21, so you score the difference: 20 points. There is no bonus, because you did not go gin, and no undercut, because Ada's count is higher than yours.
Had you waited and gone gin, Ada would not have been allowed to lay off her 7♣, and you would have scored her 28 plus a 25 bonus. Had she got down to 1 or less before you knocked, she would have undercut you and scored the difference plus 25 herself.
Hands like this are played until one player reaches 100. That player then gets 100 for winning the game and 25 for every hand they won along the way, added to the points from the hands themselves.
The hand shows the rhythm of the game: take a discard only when it fits, throw your highest loose cards early, and knock as soon as a low count makes it safe.