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The Daily Cardinal Est. 1892
Friday, May 17, 2024
Trick Candle

Ask Mr. Scientist: Silly string and trick candles

Dear Mr. Scientist,

How does silly string work? It seems like a liquid in the can, but when you spray it, it turns into a solid string.

—Brian S.

Silly string is actually pretty simple stuff. There are only three things inside the can: a plastic resin, a surfactant to help the resin foam and a propellant to shoot everything out of the can. The can is pressurized, so all of the ingredients inside are a liquid. Once you press the nozzle, the contents shoot out and go from the high-pressure environment inside the can to a low-pressure environment outside the can. Since the pressure outside the can is lower, the propellant that was once a liquid becomes a gas and evaporates. As the propellant evaporates, two things happen: The plastic resin foams and expands, and the surface of the resin dries out, creating a thin, flexible skin. In the end you are left with a really long, bendable “string” made of foam.

Dear Mr. Scientist,

How are trick candles able to relight after they are blown out?

—Sarah C.

Those magic candles that are almost impossible to blow out contain a secret ingredient: magnesium. When you blow out a normal candle, the smoke rising from the wick is vaporized candle wax. This vapor is flammable, but once the candle is extinguished, the wick is no longer hot enough to set the vapor on fire. This is where the magnesium comes in. It creates an incredibly hot flame, yet starts on fire at a fairly low temperature. Although the wick isn’t hot enough to ignite the vaporized wax, it is hot enough to ignite the magnesium, which in turn ignites the vapor and the candle relights itself.

Ask Mr. Scientist is written by Michael Leitch. If you have a burning science question you want him to answer, tweet @DC_Science or email it to

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