Gag with a Spoon

With the right mix of metals, you can make an alloy that turns to liquid at any temperature you choose

Element: Indium
Project: Prank spoons
Time: 6 hours
Dabbler | | | | | Master

Mention liquid metal, and people immediately think of mercury. After all, it is the only metal that isn't solid at room temperature. Well, not quite--it's the only pure metal, but there are many alloys (mixtures of metals) that will melt well below that point. For example, the mercury-filled fever thermometers that children were told not to play with in the 1950s and '60s have been replaced by virtually identical ones containing the far less toxic Galinstan, a patented liquid alloy of gallium, indium and tin.

Those who were kids in that era may also remember playing with another low-melting-point alloy: trick spoons that melted when you tried to stir your coffee with them. These were made with a blend that, no surprise, was highly toxic; it typically contained cadmium, lead, mercury or all three. But, as it happens, it's possible to make alloys that liquefy in a hot drink using safer components.

A few months ago I created a batch of these prank spoons as a gift for my friend and fellow element buff Oliver Sacks (author of Awakenings and Uncle Tungsten). I cast jewelers' molding rubber around a fancy spoon to form the mold. Then I looked up the formula for an alloy that would melt at 140

by Charles Shotwell

The author created the spoons by pouring molten alloy into a clear rubber mold. Next to it are the pure ingredients--from left to right: bismuth, indium and tin.Charles Shotwell

by Charles Shotwell

A steaming cup of water liquefies the spoon in about 15 seconds--notice the puddle at the bottom of the cup.Charles Shotwell