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We were now making money and the customer was happy. But the system was still vulnerable to market change: when preferences shifted and some in-flight work became "defective," we had large batches in the line. So we scrapped or reworked a lot of WIP at once—expensive and slow to react. The cost of holding and then discarding WIP in big batches made us fragile.
One-piece flow means moving and processing one item at a time (or the smallest sensible unit) instead of in big batches. Benefits: we see problems immediately, we hold less inventory, and when the market changes we have less WIP at risk—so we lose less and adapt faster. We switch to batch size one (and adjusted variance) so items move one by one. Now we're consistently making money and reacting to market changes: we listen to the customer, catch defects early, balance the line, and keep WIP low. That's Lean in action.
Try the playground next to experiment with all the levers after you've seen the full journey.
Run simulation to see Profit & Loss