Some science experiments last a few minutes. Some take years. And then there is the Pitch Drop Experiment, which has been running for nearly a century and is still not finished.

The Pitch Drop Experiment, housed at the University of Queensland in Australia, is widely known as the world’s longest-running laboratory experiment.

It began in 1927, when physicist Thomas Parnell set out to demonstrate something that, at first, sounds almost impossible: a material that looks solid can actually behave like a liquid.

A liquid that looks solid

The material at the center of the experiment is called pitch. Pitch is a black, tar-like substance that was once commonly used for waterproofing boats.

At room temperature, it looks, and feels solid. You can hit it with a hammer and it may shatter like glass. But scientifically, pitch is not a true solid. It is an extremely viscous liquid, meaning it can flow — just very, very slowly.

To prove this, Parnell heated some pitch and poured it into a glass funnel. He then let it settle for several years.

In 1930, the stem of the funnel was cut, allowing the pitch to begin its slow journey downward. Since then, the experiment has consisted of waiting for drops of pitch to fall from the funnel into a beaker below.

Why one drop takes years

That is where the experiment becomes famous. Since it began, only nine drops have fallen. The ninth drop separated in April 2014 during an operation to replace the beaker beneath it. The tenth drop is still forming.

The reason it takes so long is viscosity. Viscosity is a measure of how resistant a fluid is to flowing. Water has low viscosity, so it pours quickly. Honey has higher viscosity, so it flows more slowly. Pitch is on another level entirely.

The University of Queensland describes pitch as the world’s thickest known fluid, and Guinness World Records notes that the experiment has shown pitch to be about 100 billion times more viscous than water.

This is why the Pitch Drop Experiment is so useful as a teaching tool. It challenges our everyday idea of what solids and liquids are. We usually judge materials based on what we can see over a short time. If something does not move in front of us, we call it solid.

But science often works on timescales far beyond human patience. A mountain may seem permanent, but it can erode.

Glass may look rigid, but materials can behave differently under different conditions. Pitch shows that some substances can appear solid in daily life while still flowing over long periods.

A century-long lesson in patience

The experiment is also famous because of its strange history of near-misses. For decades, no one actually saw a drop fall in person. Drops fell at inconvenient moments, or cameras failed to capture them.

The eighth drop fell in 2000, and the ninth separated in 2014. Today, the experiment can be watched online, turning one of the slowest events in science into a kind of global waiting game.

What makes the experiment remarkable is not technological complexity. It is actually very simple: pitch, a funnel, a beaker, and time. But that simplicity is exactly why it works.

It makes an invisible idea visible. It turns viscosity from a textbook term into something you can understand with one image: a drop that takes years to fall.

The Pitch Drop Experiment is not about speed, spectacle, or instant results. It is a reminder that nature does not always move at the pace humans expect. Some truths reveal themselves only when someone is patient enough to keep watching.

Nearly 100 years after it began, the experiment continues. And somewhere inside that glass funnel, the next drop is slowly making its way down.