Desktop toy of the day: The Powder Toy
An extremely comprehensive, but nonetheless easy to play with, physics sandbox into which you can throw a variety of solids, liquids, powders, gasses, manipulate forces, and more.
If you've ever played sand:box on Android or Steam, you'll be familiar with the genre.
The Powder Toy is libre, longer established, and does more.
Download it from the project website:
https://powdertoy.co.uk/
Build it from source:
https://github.com/The-Powder-Toy/The-Powder-Toy
Or grab it from Flathub:
https://flathub.org/apps/uk.co.powdertoy.tpt
