If you spend your mornings sourly contemplating the tea you spill on your counter - no matter how carefully you pour - you're not alone. Nearly everyone spills a few drops. And despite talented physicists trying to figure it out, no one knows why.
Some problems never get solved because they're difficult. Some never get solved because everyone already assumes they know the answer. The mystery of the Teapot Effect remains unsolved for both those reasons. Experiments have been done to determine why some of the tea dribbles down the spout even when the tea is poured expertly, but the experiments haven't been conclusive. More importantly, for a long time, no experiments were done because everyone was sure they knew why their tea got spilled.
The answer, they believed, was adhesion. Certain molecules adhere to each other. We see this in the meniscus - the little upward curve that water makes when it clings to the sides of a test tube or a glass or a cereal bowl. Water molecules follow the curve of the spout of a teapot, apparently because they adhere to ceramic tea pots. And also silver tea pots. And occasionally glass tea pots.
Physicist Markus Reiner didn't believe the hype. He and a fellow scientist did a simple experiment, coating the spout of a tea pot with paraffin wax. The tea still spilled. So much for adhesion. But what really convinced him was a series experiments inspired by dissolving salt crystals in the Dead Sea. When salt crystals dissolve, they release salt into the surrounding (already salty) water, making it even more salty. This ultra-salty film of water was denser than the surrounding water and so sank downwards, but as it flowed down, it clung to the salt crystal, follow its angles rather than dropping straight down.
So? Salt molecules stick together. Salt molecules also seem to stick to an Erlenmeyer flask. Reiner slowly injected a stream of salt water next to the flask as the flask stood in fresh water. As the salt water ran down the sides, it stuck, even when the sides of the flask curved and the water should have flowed straight down.
Lastly, Reiner stuck a flask in salt water and injected fresh water under the bottom of the flask. Because fresh water is less dense, it flows upwards in salt water. But it still clung to the sides of the flask. Hot water injected in cold water will cling to the flask. Cold water injected in hot water will do the same. As long as it's flowing over the solid object, there seems to be some cling that doesn't have to do with the adhesive properties of the individual molecules.
The Teapot Effect remains unsolved, and our kitchen tables remain splattered with tea. But at least we know that we don't know why it happens.