Click the box (or press Space) to arm it. It stays gray, then turns red for a random wait of 1–5 seconds — don't click yet. The instant it turns green, click as fast as you can. Click too early and it's a false start.
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A simple visual reaction time is the sum of several stages: light hits the retina, a neural signal travels to the visual cortex, the brain decides "go," and a motor command reaches your finger. In young adults this typically totals about 200–250 ms. This test also includes your browser's own display and input latency (screen refresh, event loop, click hardware), so measured times here usually run a bit higher than a dedicated psychology lab's equipment would report.
If the wait were always the same length, you could learn to anticipate exactly when green appears and time your click to that rhythm instead of truly reacting to the light — measuring rhythm prediction, not reaction time.
Drawing the wait uniformly from 1–5 seconds removes that predictability, so a fast score actually reflects your sensorimotor response speed. Clicking during the red phase (a "false start") is logged separately and excluded from your averages, the same way a psychology experiment would discard anticipatory responses.