In 1982, Lucasfilm's computer graphics group had to set a whole planet on fire for Star Trek II: The Wrath of Khan. Computers of the day drew surfaces, and fire has none. So William Reeves stopped trying to draw fire and grew it instead: thousands of dots of light that are born, fly, cool and die. Each dot is mostly red with a touch of green, and no dot hides another. They add their light. Where enough of them pile up, red hits its maximum, green keeps climbing, and the colour turns yellow, then white. Nobody painted the hot heart of that fire. It painted itself. In the frames where the wall of fire swallows the camera, Reeves' paper counts about 400 particle systems and over 750,000 particles. In 1986 his graphics group became Pixar.

The fire in this video is our own recreation of Reeves' method, not footage from the film.

Which movie effect should we take apart next?

Sources: William T. Reeves, "Particle Systems: A Technique for Modeling a Class of Fuzzy Objects", ACM Transactions on Graphics, April 1983 (Lucasfilm); Pixar, Our Story.
Sounds: Freesound, CC0 (shaunhillyard, lfyaudio, 15GKovacovaJulie).