You are learning quantities in shanghai: you will read the scene it happens in, learn 10 words and phrases, take apart the pattern behind them (量词 + market quantities), hear them in a conversation, rehearse them, and finish by saying one thing out loud to be graded.
Shanghai markets use Chinese weight units: 斤 (jīn, 500g) and 两 (liǎng, 50g). You need to know how to buy the right amount and adjust quantities — more or less.
The market scale is a digital device, but the vendor still hovers over it — watching to see the customer's reaction, ready to add more or remove some. This negotiation of weight is one of the most human transactions in a Shanghai market.
The stranger is buying hairy crabs. These are sold by weight, individually assessed for liveliness. The vendor holds one up — it snaps its claws. 「这只多少钱？」 The vendor places it on the scale: 260g, ¥95. 「来两只。帮我选好的。」
The vendor goes through the tank, picking up crab after crab, squeezing the shells gently, checking claw strength. He picks two. The stranger watches. 「称一下。」 Total: ¥178.
「用支付宝。」 The QR code appears. Scanned. 「谢谢！」 The crabs go into a mesh bag, legs still moving. The freshest purchase possible.