Powdertron

Four simulators. One sandbox.
Powdertron is a physics playground where the chemistry is real. Its 674 materials — pure elements, compounds, alloys, minerals and industrial stock — each carry a genuine density, melting point, boiling point and formula, and 1,409 reaction rules decide what happens when they meet. Iron rusts. Sodium explodes in water. Limestone calcines. Nothing here is a lookup table of pretty colors.
Chemistry
Paint sand, water, acid, thermite or plasma onto a canvas of up to 120,000 cells and let it run. Heat it, freeze it, set it alight, pull the air out to a vacuum or switch gravity off entirely. Phase changes, combustion, dissolution, radioactive decay, heat conduction and structural collapse all play out cell by cell in a multithreaded C++ engine — fast enough that a full canvas still runs smoothly.
Electronics
A real circuit simulator, not a wiring puzzle. Batteries, resistors, capacitors, inductors, logic gates, seven-segment displays and an oscilloscope, connected by actual nets rather than cell adjacency. Build something that works, package it into a named chip, and drop that chip into your next design.
Wind Tunnel
Draw a shape, put it in the airflow, and see what happens. A lattice-Boltzmann solver renders live streamlines, pressure across the surface of the body, and the point where the flow separates.
Optics Bench
Lenses, mirrors and prisms with wavelength-accurate refraction. Split white light into its spectrum, chase a focal point, or build something that shouldn't work and find out why it doesn't.
Learn it by breaking it
Every material carries its real formula and properties, and reactions you trigger are logged as discoveries as you find them. A guided tutorial and a set of story challenges give you somewhere to start if a blank canvas is too much choice.
Build together
Host a session over Steam and share a canvas with friends. Everyone paints into the same world, sees each other's cursors, and watches the same simulation run. Name your worlds, give each its own physics settings, and lock the session with a password if you'd rather keep it private.
Make it yours
Add your own materials through the mods folder, save and load worlds, capture screenshots, and export measurement data straight to CSV for when the sandbox turns into an experiment.