The Science of Fortune-Telling Fish: A blast from your past.

[Images via x & x.]

Most of us remember at one time or another, during summer camp, or in science class, getting those flimsy red fish that told you your fortune, or at least a generalized summary of your current personality. Kids, and adults alike, captivated by a flailing cellophane red fish in the palm of their warm hands. 

So what makes them work? Some people might think it’s due to body heat, or even electrical currents, but instead it’s a surprisingly simple solution based on the fish’s chemical composition.The cellophane used to make the fortune telling fish is a hygroscopic material, which simply means water-finding material. It is made of a polymer called sodium polyacrylate, the molecules of the sodium polyarcylate absorb water, or sweat from the user’s hands, and this changes their shapes. As the fish absorbs the water it moves through very small pores in the cellophane by a process that is known as capillary action. The cellophane itself swells due to this reaction which causes the fish to curl up and even “dance” or “wiggle” because it is super light and also very susceptible to the slightest air currents. 

Of course everyone’s hands are different, so the process happens differently for each person, influencing the myth of distinct ‘fortune telling’. Because the sodium polyacrylate absorbs moisture so easily, if you were to place it in water, and then try to use it on your hand, it wouldn’t work. But, if you dried it out, and tried it again, it would work perfectly, thus rendering it reusable, which is great when you’re trying to keep some kids busy! 

Fun fact: Sodium polyarcylate is the same thing used to make diapers so absorbent.

Take It Further via Steve Spangler Science:

Now that you understand the scientific properties behind Fortune Telling Fish, you can conduct some experiments with them.

• Try setting Fortune Telling Fish on different areas of your body like the tops of your hands, your arms, or even the tops of your feet. The difference amounts of sweat glands and moisture can have direct effects on how the fish react.
• Do you think the fish will react differently if your palms are a different temperature? Test the Fortune Telling Fish on your hands after holding them in a freezer or in front of a space heater.
• Test different surfaces to see if Fortune Telling Fish react on them. Who knows? Maybe the fish will pull moisture from countertops, desks, or keyboards.