When a crystal forms it locks its uranium in, and the uranium starts halving on a schedule nothing can disturb — not heat, not pressure, not price. Nobody sees the clock start; geologists read the parent-to-daughter ratio and recover the age anyway.
Bright grains are parent atoms; dark ones are what they decayed into. The measured age comes from the ratio alone — watch it track the true clock. Click the grain to crystallize it fresh, drag the slider to change the half-life, arm the ticks to hear each decay.