Simulations

Send fluid through a pipe that narrows to a throat and widens back out — adjust the inlet/throat diameters, flow speed, fluid density, and inlet pressure to watch tracer particles speed up in the throat while manometers show the pressure drop Bernoulli's equation predicts.

Trace the curve that explains where nuclear energy comes from — see why fusing light nuclei and splitting heavy ones both release energy, and check the energy of a specific reaction against the curve.

Watch an electron drop between energy levels of a hydrogen-like atom and see the photon it gives off — pick the levels and the nuclear charge, and read the wavelength, energy and which series the line belongs to.

Drop a uniform object into a tank of fluid — adjust the object's density, the fluid's density, and the object's size to see it settle at its equilibrium floating depth, or sink straight through, exactly as Archimedes' principle predicts.

Follow the Moon through a month of phases from the Sun, Earth and Moon's positions, and see it from the Northern or Southern Hemisphere. The half of the Moon facing the Sun is always lit; what changes is how much of it we can see. Then find out why most new and full Moons bring no eclipse, because the Moon's path is tilted, and when one does.

Start with a sample of undecayed nuclei — adjust the initial count and half-life to watch the population (and a grid of individual nuclei) decay exponentially.

The server picks a starting number of nuclei and a check time, and a target for how many should still be undecayed then (always achievable). You choose the half-life, and a real radioactive-decay model counts what is left. Hitting the target refunds your run.

Watch Earth go once round the Sun with its axis tilted, and see how the tilt, not the distance from the Sun, makes the seasons. Pick a latitude and a tilt, then follow how long the Sun is up each day and how high it climbs at noon through a whole year. Near the poles the Sun stays up all day in summer and never rises in winter. Set the tilt to 0° and the seasons disappear. The two halves of the world have opposite seasons.

Punch a hole in the side of a water tank and watch the jet arc out and the level fall — the jet speed depends only on how deep the hole is below the surface, and the tank empties on a curve you can predict.

The server picks a water tank, the width of its hole and a bucket on the floor, plus a moment in time (always achievable), you set the height of the hole, and a real draining-tank model judges whether the jet is landing in the bucket at that moment. Landing it in the bucket refunds your run.