The magic of the movies

"How about a little fire, Scarecrow?" The Wicked Witch of the West taunts a terrified Scarecrow with her flaming straw broom in "The Wizard of Oz" (1939) – and then sets him on fire. But how could she? Wouldn't the actor get hurt? Not likely. This scene, like many in the movies, was accomplished with film tricks known as special effects.

"The actor [Ray Bolger] probably had an asbestos sleeve on one arm," explains Andrew Poleszak, a costume designer in Boston. "They padded out the arm of his costume, put combustible fluid on the cloth, and set the edges on fire." Dorothy doused the Scarecrow with water to squelch the flames, which "melted" the Wicked Witch, played by Margaret Hamilton. For this effect, Hamilton stood on a small hydraulic lift that was lowered into the floor. Dry-ice fog made it seem as though she was melting, and her long black skirt hid the trick.

The purpose of special effects – in theater, on television, or in the movies – is to create illusions that are believable, no matter how fantastic: a boy wizard flying through clouds, a tornado scooping up a farmhouse, or an intergalactic spaceship racing through infinite space.

How do they do all that?

Before modern technology, such as computers, special effects involved some unusual ideas. When director Cecil B. DeMille needed to create the illusion of Moses parting the Red Sea in "The Ten Commandments" (1923), he poured thousands of gallons of water down the sides of a U-shaped tank. DeMille filmed the action as the torrents crashed together – and then ran the film backward to get the desired effect. (The actors were added later.) To stage elaborate car chases by a wacky gang of early-20th-century movie characters called the Keystone Kops, movie crews dumped liquid soap on real streets and hired stunt drivers to skid their cars through the slippery mess.

Moviemakers also combined painted backdrops with live-action sequences to make scenes more dramatic. The tornado that rips Dorothy's house from the Kansas prairie in "The Wizard of Oz" was constructed from muslin (a strong fabric) built into a large cone shape. A machine suspended from the ceiling of the studio moved the fabric, and colored talc (called fuller's earth) was blown through the fabric with a hose to create a dust storm. Once the tornado action was filmed, the crew shot the background of a stormy sky (large panes of glass with painted cotton for clouds). This was then projected behind Judy Garland, who played Dorothy. Wind machines added realism to the scene.

"You simulate through machines what Mother Nature does," says Douglas Weber of Theatre Effects, a special-effects business in Cincinnati. Mr. Weber says that he uses all the science, math, and English he learned in school in his special-effects work. "If someone needs a lightning effect, you have to understand how lightning happens. All those things in school I never thought I'd use – there's nothing that I learned that I don't use now."

For "Pirates of the Caribbean: The Curse of the Black Pearl" (2003), Weber and his colleague Chris Wyllie cooked up an ominous fog to fill the screen when the Black Pearl pirate ship first appears. They also created the fog that surrounds Jack Sparrow and crew as they row into a cave where pirates' gold is stashed. "We created a wispy haze you can see through," says Weber, "so the actors weren't obscured." His and Mr. Wyllie's fog concoctions were made of what they call "people-friendly chemicals."

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