Pack

one of them answers the light differently.

tear the foil, thumb the stack, move light across each card -- the rare one shifts colour, not shine

A metallic card is a mirror: its highlight slides as you tilt it, but stays one colour. A holographic card is a diffraction grating: ridges send a different colour at each angle. Opal does the same trick naturally.

Concepts: diffraction, reflection, light, rarity.

An interactive you play with one thumb, on Wundr.