A 20 m water-filled conduit is heated from below. Watch deep water become superheated, flash into steam and throw the column out; then it refills and the cycle repeats.
The science
Boiling point depends on pressure. Water at pressure p boils at Tsat(p) (IAPWS-IF97 saturation line). At the surface p = 101 kPa gives 100 °C; 19.75 m of water adds ρ g h = 1.86 bar and raises Tsat at the bottom to about 132 °C. Cells are therefore stable even when hotter than 100 °C.
Heating and mixing. The conduit is 40 cells of 0.5 m (area 0.25 m², ρ = 958 kg/m³). Heat Q warms the lowest 8 cells: dT = Q dt / (n m cp), cp = 4186 J/(kg K). Turbulent mixing is a diffusion term with κeff = 0.02 m²/s. Cells shallower than 5 m boil off gently (they cannot erupt).
Trigger and cascade. When a cell reaches T ≥ Tsat(p) the water above it is thrown out; with less overlying water the pressure falls, so cells below may now exceed their (lower) Tsat and flash too. The cascade stops at the first stable cell.
Energy and jet. Flash energy E = Σ m cp(T − 373.15 K) makes steam of mass E/Lv (Lv = 2.257 MJ/kg). The jet speed is v = √(2 η E / Me) and the height H = 0.8 v²/(2g) (0.8 = rough drag factor). η = 1.5 % is a tuned model constant chosen so that default eruptions are tens of metres high; it is not a measured value.
Simplifications. 1-D column, no dissolved gases, no side channels, no pre-play bursts, constant water density, instantaneous eruption (the jet duration drawn on screen is presentation only). Not a model of any real geyser; do not approach real geysers - they are dangerous. Models built with Blender, scene by Three.js.