The easiest way to make a geyser at home is the classic Mentos and Diet Coke experiment, which can launch a fountain of soda 15 to 20 feet into the air in seconds. All you need is a two-liter bottle of diet soda, a roll of Mentos mint candies, and a flat outdoor space you don’t mind getting sticky. The science behind it is surprisingly elegant, and getting the biggest eruption comes down to understanding why it works.
What You Need
Keep the supply list simple:
- One two-liter bottle of diet soda. Diet Coke produces the best results, though other diet sodas work. Diet outperforms regular because it lacks sugar, which thickens the liquid and slows bubble formation.
- One roll of Mentos mint candies. The original mint flavor works best. Fruit varieties have a smoother, waxier coating that reduces the effect.
- A piece of paper or cardboard tube. You’ll use this to drop all the candies in at once. Roll an 11-by-17-inch sheet of paper into a tube that fits snugly over the bottle opening, or use a paper towel roll cut to size.
- Safety goggles. Soda in the eyes stings. The American Chemical Society recommends eye protection for this experiment.
Do this outside on a flat surface where soda spray won’t damage anything. Grass, a driveway, or a parking lot all work. Wear clothes you don’t care about.
Step-by-Step Setup
Set the unopened bottle on a flat, stable surface. If it’s a windy day, you may want to wedge it between rocks or shoes so it doesn’t tip. Open the cap and set it aside.
Stack five to seven Mentos in your paper tube. Hold a flat piece of cardboard or an index card over the bottom of the tube to keep the candies from falling out. Position the tube directly over the bottle opening, pull the card away, and step back fast. You have about one to two seconds before the eruption starts. The key is getting all the candies into the bottle simultaneously. Dropping them one at a time produces a much smaller fountain because fewer candies are reacting at once.
If you want a hands-free option, some people rig a simple string-and-pin mechanism: thread a pin through the paper tube to hold the candies in place, position the tube over the bottle, then pull the pin from a few feet away using a length of string. This gives you more distance from the blast.
Why It Erupts
This isn’t a chemical reaction. It’s a physical process called nucleation. Carbon dioxide is dissolved in the soda under pressure. When you open the bottle, the gas wants to escape, but it needs a surface to form bubbles on. Normally, CO2 leaves soda slowly because the inside of the bottle is smooth and gives gas molecules few places to gather.
A Mentos candy is the opposite of smooth. Under a microscope, its surface is covered in tiny pits and bumps roughly 1 to 3 micrometers across. A single Mentos candy has between 50,000 and 300,000 of these microscopic nucleation sites, each one acting as a launchpad for a new bubble. When the candy hits the soda, tens of thousands of bubbles form almost instantly.
The candy’s coating also contains gum arabic, a natural ingredient that acts as a surfactant. Surfactants lower the surface tension of the liquid, which reduces the energy needed to form a bubble. So the Mentos is doing two things at once: providing a massive number of places for bubbles to form and making it easier for those bubbles to form in the first place. The result is an explosive release of CO2 that pushes the soda out of the narrow bottle opening as a high-speed jet.
Tips for a Bigger Eruption
Temperature matters. Warmer soda releases gas more readily than cold soda, so a bottle that’s been sitting in the sun for 20 minutes will erupt higher than one straight from the fridge. Don’t heat it artificially, just let it come to room temperature or slightly above.
Use the full roll. More candies means more nucleation sites hitting the soda at the same time. Seven Mentos will outperform three. Some experimenters use a full roll of 14, though returns diminish after about seven or eight because the bottle opening limits how fast gas can escape.
Don’t shake the bottle beforehand. Shaking releases some CO2 early, which means less gas is available for the main eruption. You want as much dissolved carbon dioxide as possible still trapped in the liquid when the candies drop in. For the same reason, open the cap gently and work quickly.
Bottle shape plays a role too. The standard two-liter bottle has a narrow neck that focuses the escaping soda into a tight column. Wider-mouthed containers produce a less impressive fountain because the gas can escape without building as much pressure.
How Natural Geysers Work
If you’re curious about real geysers, the basic physics share some DNA with the soda experiment, but the energy source is entirely different. A natural geyser requires three things: a water supply, a heat source, and a specialized plumbing system underground.
Deep below the surface, groundwater seeps into fractured rock near a heat source, typically magma or hot volcanic rock. The water gets superheated well beyond its normal boiling point, but the immense pressure from the rock and water above keeps it in liquid form. Steam and bubbles gradually accumulate in side chambers and cracks, building pressure like a loaded spring. When that pressure finally overcomes the weight of the water column above, the system decompresses rapidly. Superheated water flashes to steam, expanding violently, and the eruption blasts water and steam out of the surface vent.
According to the U.S. Geological Survey, geyser eruptions are driven by the conversion of thermal energy to kinetic energy during this decompression. The water doesn’t just boil; it essentially explodes upward as the pressure holding it down suddenly drops. This is why geysers need that specialized underground plumbing: a simple hot spring lets steam escape gradually, but a geyser’s narrow, constricted channels trap pressure until it releases all at once.
You can’t replicate a true geyser at home safely, since it requires superheated water under extreme pressure. But the Mentos experiment captures the core principle: a rapid release of dissolved gas through a narrow opening, producing a dramatic vertical fountain. It’s the same basic physics, just powered by candy instead of magma.