Plumb-line — free browser game

Plumb-line is a decision game that trains your eye to find a signal hidden in noise. The scene is simple: as a stonemason's apprentice you've hung a plumb-bob next to a wall, and the wind is swinging it. The physics is the key: however wide its swing, the time-average position of a plumb-line is the true vertical. Your job is to spot that average by eye and decide whether the wall behind it is parallel to that vertical (plumb) or leaning slightly to one side. Each round the wall's tilt is tiny (about ±1°-3°) while the wind sways the bob 10-50 pixels away from vertical — trusting the bob's momentary position will mislead you; you have to find the centre of its swing. There are three buttons: ← (Leans Left), ↑ (Plumb), → (Leans Right); the keyboard equivalents are the arrow keys or A/S/D, and Space submits 'Plumb'. A correct call awards +60 points plus a time bonus from the seconds you had left plus a streak bonus; a wrong call costs one apprentice mark (life) and resets the streak. Three lost marks ends your apprenticeship, and your best run is saved in the browser. Every four correct calls bumps you to the next level: stronger and noisier wind, smaller wall tilts (harder to discriminate), and a slightly shorter time window. Tip: as the bob swings, two extreme turn-points appear — their midpoint is the true vertical. A single glance won't do it; watch at least two full swings, then shift your eyes to the top of the wall and ask 'which side of the midpoint does the wall pass on?'. The wind gauge sways left-right; even at peak wind the true vertical doesn't change — only the bob's momentary position does.