Fire silver atoms through a magnet and they never smear across the screen — they land in exactly two spots. That two-valuedness is spin. Prepare a state with the first magnet, block one beam, then tilt the second magnet: the split obeys P(↑) = cos²(θ/2) — the angle alone. Cross the analyzers at 90° and it is always exactly 50/50: measuring x has erased z.
The which-path sibling of the double slit — there, looking destroys interference; here, measuring one axis destroys another's purity. See the double slit →
Hit Fire one a few times — each atom flips a Born-rule coin and sticks to one of two piles. Stream and the piles grow ragged, then smooth toward the theory line. Drag the bright arrow on the Bloch sphere to re-prepare the state, or tilt Dial B: at θ=0 it's 100% up, at θ=90 always 50/50, at θ=180 fully flipped. The grey ghost band is what a classical dipole would smear across — the gap the two-spot data refuses to fill.