HOW COME? If you tilt a candle sideways, its flame always points up

If you tilt a candle sideways, why does the flame turn and keep pointing up? asks a reader.

Think of the flame on a candle as a helium-filled balloon: No matter which way you hold the string or the tied-off end, the balloon will stubbornly right itself. It's as if candle flames and party balloons took Mom's advice a little too seriously: Stand up straight!

In fact, turn a candle upside down, and its flame will shift around to point to the ceiling, as if pulled by an invisible force.

The result is a torrent of hot wax, as the flame envelops a now-dripping candle.

From the moment the predecessors of modern humans began to use fire and make simple torches, we have probably wondered about the strange behavior of flames. The 19th-century British scientist Michael Faraday did a lecture series aimed at young people in London, and six of the talks were about candles. "There is no more open door by which you can enter into the study of [science] than by considering the physical phenomena of a candle," he said, going on to discuss what makes a candle flame bright, how it burns, why flames come in different colors, and why a burning candle produces water vapor.

A candle flame keeps pointing up because it heats the surrounding air, creating streaming currents around itself. Warmer air is thinner and lighter than cooler air, since gas molecules spread out when they gain energy. As the heated air near a flame expands, it also rises, since it's lighter than the air around it. Cooler air rushes in to replace it, heats up and likewise rises, in a continuous stream.

When you look at a burning candle, you can't see this "convection current" around the flame. But it's this up-rushing air stream that keeps the flame pointing skyward. The hot gases, rising toward the ceiling, also give the flame its arrow shape.

So no matter how we move the candle, the flame's gases continue to rise up and up, and the flame follows. Just as a party balloon's lighter-than-air helium pulls it toward the ceiling, or a hot-air balloon points up to the sky, no matter how its basket is tilted before takeoff.

But a flame acts (and looks) very different in a place where gravitational forces are very tiny, such as in the free-fall environment of a space station. Experiments on the space shuttle showed that in conditions of "microgravity," a candle flame loses its familiar feather shape and changes color. The air near a flame is still heated, but there are no real currents. The result is a slower but hotter burn, and a blue flame, shaped like a dome.