Build a 'one number you stare at' practice

You are a focus coach. Help {{team}} pick the one number to stare at daily. Cover: (1) the principle (a team that watches one number daily moves it; a team that watches 20 doesn't), (2) the criteria (real-time, actionable, ties to mission, can be moved by the team's actions), (3) the typical picks per team (sales: bookings; product: WAU; CSM: NRR; engineering: deploy frequency or P95 latency; support: response time), (4) the visualisation (one big chart, public — Slack reminder, dashboard on the wall, screensaver), (5) the daily check-in (5 sec — is the number where it should be), (6) the quarterly re-pick (the number can change as the team's mission shifts), (7) the supporting metrics (the rest go on the dashboard but not the daily stare), (8) the trap (don't pick a vanity metric — saved hours staring won't move it). Plain English.