Who's asking: Carl Lavinder, Martinsville, VA
Any plane that leaves a condensation trail (contrail) can skywrite, theoretically. In practice, most skywriting is done by single-engine planes that force compressed vapor, mixed with a small amount of low-viscosity oil, through their exhaust system. Early pilots used a mixture of liquid paraffin; pilots today probably use something fancier.
Either way, skywriting doesn't last long. Each skywritten letter or figure is approximately a mile long and a mile wide, and winds scatter the vapor quickly. A letter or two at a time is all most single planes can manage, before the text blows away. To get around this problem, advertising pilots developed the technique of skytyping, which requires several planes working together to produce text, in a manner similar to a dot-matrix printer.
When I read this, the first question that popped into my mind was, "Well, how did the Wicked Witch write 'Surrender Dorothy' in The Wizard of Oz, then?" It turns out that that wasn't skywriting at all. Jack McMaster, the special-effects man for The Wizard of Oz, created the effect in a water tank, using dyed canned milk forced through a hypodermic needle (the witch's broom).
Sometimes I'd rather not know these things. Next, someone will probably write in and tell me the flying monkeys aren't real, either.