Clog

slide the pebble. keep the rice pouring.

a funnel of rice that keeps jamming, and a pebble on a thread you can lower over the spout

A jammed funnel is an arch: a few grains lock into a bridge resting on the edges of the hole and carry the load above. A stone at the right height takes that load and leaves an arch nowhere to stand, so it jams about half as often; too high it shields nothing, too low it pinches its own arch. Shaking works because it breaks the bridge.

Concepts: granular-flow, jamming, force-arches, friction, fluid-flow, obstacle-assisted-flow.

An interactive you play with one thumb, on Wundr.