How does a candle burning at both ends rock like a perpetual-motion drinking bird?

Here’s how a candle burning at both ends rocks like a perpetual-motion drinking bird.

Grab a candlestick, a match, and a pin, and let’s do the impossible: burn the candle at both ends.

First, make sure you’re working on a clean, flat surface like a bare kitchen counter or table—one that won’t instantly go up in flames if the candle tips over, and one that also won’t get ruined if splattered with wax droplets.

You’ll need a needle, pin, or round, smooth toothpick; two sturdy ceramic or glass cups; and a lighter, along with your regular-sized candle (but not one of those dripless kinds).

Cut or melt the wax at the bottom of the candle so that both ends have exposed wicks to burn. Stick your pin all the way through the center of the candle, perpendicularly and an equal distance from each end.

Balance the ends of the pin between the cups, one end on each lip. The candle should be horizontal, dangling in the space between the two cups. Now light both ends and gently get it rocking.

It won’t stop rocking as long as both flames stay lit.

What happens is this: Each time a drop of wax falls, the candle pulls up at the same time, —it’s a working model of Newton’s law about an action creating an opposite and equal reaction.

The weight of the candle also slightly shifts as that end suddenly gets lighter, helping to keep the candle rocking in a back-and-forth motion.