Physicists Can Finally Explain How Sand in an Hourglass Can Suddenly Stop Flowing

<img width="128" height="52" src="https://www.sciencealert.com/images/2024/04/sand_hourglass_pouring.jpg" alt="sand in an hourglass"/>
It's not just about sand.

https://www.sciencealert.com/physicists-can-finally-explain-how-sand-in-an-hourglass-can-suddenly-stop-flowing