Sand does not drain like a liquid: grains jam and rearrange, so the discharge rate through a narrow opening depends only on the size of the opening and the grain diameter — not on how much sand is piled above it (unlike a liquid, where pressure head matters). This is captured by the Beverloo equation, the standard empirical law for granular discharge through an orifice:
Q = C·√g·(D − k·d)1.5
where D is the neck width, d is the grain diameter, k≈1.4 accounts for the empty-annulus effect near the wall (grains can't use the full opening), and C≈0.58 is a discharge coefficient fitted from experiment. Widening the neck slider raises D and grows the flow rate along that 1.5-power curve; narrow it below k·d and the opening jams completely — no flow at all, exactly as with real sand.
Each grain here falls individually under gravity, bounces off the funnel walls with damping, and settles onto the growing pile at the angle of repose — the steepest slope loose sand can hold before it slides, typically 30–35° for dry sand — which is why both the draining top surface and the accumulating bottom pile keep their characteristic conical shape.