Fire a particle through the slit. The box sets how tightly you pin its position (Δx) — but squeeze too tight and its momentum (Δp) runs wild, fanning the shot out. Loosen it and the shot stays straight but its starting point smears.
Find the balance where the beam is narrowest, aim it at the slit, and fire.
Δx · Δp ≥ ℏ/2 · ▒