Can Sugar Water Make Things Float!? The Hidden Secrets of Buoyancy Behind Conan’s Trick and the Dead Sea
I’m science trainer Ken Kuwako. Every day is an experiment.
【This article is also available as a radio talk!】
“Why don’t ships sink?”
“Why does ice float on water?”
At some point as kids, we’ve all wondered about questions like these. Surprisingly, the answers are hidden inside just two scientific ideas: density and buoyancy. Today, let’s uncover the mystery together through fun experiments, everyday examples, and even tricks from famous manga stories.
What Is Buoyancy? Archimedes’ Brilliant Discovery
Remember the strange feeling when you jump into a swimming pool and suddenly feel lighter? That sensation is buoyancy in action.
Whenever an object is placed in a liquid, the liquid pushes upward on it. This upward force is called buoyancy. According to Archimedes’ principle, the strength of that force is equal to the weight of the liquid displaced by the object.
The ancient Greek scientist Archimedes is said to have discovered this while taking a bath. Legend says he became so excited by the realization that he ran naked through the streets shouting, “Eureka!” (“I found it!”). It remains one of the most famous stories in science history.
Let’s look at it a little more closely. Suppose an object has volume V and density ρ. The gravitational force acting on it can be written as ρVg (where g is gravitational acceleration). If the object is completely submerged in water with density ρ’, the buoyant force becomes ρ’Vg according to Archimedes’ principle.
Whether Something Floats or Sinks Depends on Density
In other words, whether an object floats or sinks depends on the balance between gravity and buoyancy.
If the object’s density is greater than the density of water, gravity wins and the object sinks.
ρVg ＞ ρ’Vg
ρ ＞ ρ’
On the other hand, if the object’s density is lower than the density of water, buoyancy wins and the object floats.
ρVg ＜ ρ’Vg
ρ ＜ ρ’
That’s the secret: the key to floating and sinking is the difference in density.
A Magical Floating Experiment with Sugar Water
Once you understand density differences, you can create experiments that look almost like magic. Take a look at this video.
In the video, a water balloon resting at the bottom of a tank slowly rises upward as a mysterious liquid is added little by little.
So what’s happening here?
The “magic liquid” is actually sugar water packed with dissolved sugar. Sugar water is denser than ordinary water. As more sugar water mixes into the tank, the density of the surrounding liquid gradually increases. Eventually, it becomes denser than the water balloon itself, causing the balloon to float upward.
You can do a similar experiment using eggs, which makes the idea feel even more familiar. But the water-balloon version has one big advantage: at the start, the liquid inside and outside the balloon has nearly the same density, making it especially easy to understand how changing density makes objects float.
I often perform this experiment in front of students, and every time it gets a huge reaction. It’s one of those demonstrations that never fails to amaze people.
Of course, the ultimate buoyancy experience would be visiting the Dead Sea. Its salt concentration is over 30%—about ten times saltier than ordinary seawater. Because the water is so dense, people can effortlessly float on their backs without even trying. It’s basically Archimedes’ principle you can experience with your entire body.
Even Detective Conan Used a Buoyancy Trick!
Buoyancy isn’t just for science class—it also appears in fiction. The popular manga 『Detective Conan』 features an interesting trick based on buoyancy in volumes 82–83.
In the story, Conan investigates a murder case at a mountain lodge. A tomato sunk at the bottom of a bathtub suddenly rises to the surface after something is added to the water.
The manga mentions bath additives, but the explanation reveals the real trick: salt. By increasing the density of the surrounding liquid, the tomato becomes less dense than the liquid around it and floats upward.
It’s a fantastic example of real science being cleverly woven into a mystery story.
Source: 『Detective Conan』 (Shogakukan), Gosho Aoyama, Volume 83
The Mystery of Floating Ice — Water Is a Very Special Substance
Here’s another fascinating example from everyday life: water itself.
When water freezes into ice, something unusual happens. Unlike most substances, ice is actually less dense than liquid water.
That’s why ice cubes expand and bulge upward in an ice tray when frozen.
For example, water at 0°C has a density of about 0.9998 g/cm³, while ice at 0°C has a density of about 0.9168 g/cm³. Because ice is less dense, it floats.
This strange property is incredibly important for life on Earth.
If ice were denser than water, lakes and rivers would freeze from the bottom upward in winter, turning completely solid. Fish and aquatic life would struggle to survive. But because ice floats, only the surface freezes while liquid water remains underneath, protecting living creatures below.
In a very real sense, this peculiar property of water helps sustain life on our planet.
Turn Everyday “Why?” into “Aha!” Moments
Science is full of hidden wonders waiting to be discovered in everyday life. Once you understand the relationship between density and buoyancy, so many everyday mysteries suddenly make sense.
The sugar-water experiment, the floating egg experiment—these are all simple activities you can try at home with common materials. Give them a shot yourself! Experiences you feel and see firsthand often become the kind of scientific knowledge you never forget.
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キーンと冷えるドライアイス！気温が上がってくるこの時期・ドライアイスを使った昇華・凝結・等速度直線運動の実験はいかが？
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