Pour some clear water into contaminated water, and the contaminated water gets a little less densely contaminated. But nothing happens to the clear water, right? Wrong. The clear water gets contaminated as well, because particles can travel up waterfalls.
In Cuba, scientists like to relax with a nice cup of tea. They do this by decanting water into containers that have yerba mate tea in them. To their frustration, they found that afterwards they not only had to clean out the container with the tea leaves, but also the decanter that had only held water. Some would account for this by saying that they were careless and got some tea on their hands, and then touched the water container. The scientists took a closer look and found out that the tea particles could not only travel upstream, but travel against the force of gravity as well. They moved up and into the clean container.
The particles can't leap upstream like salmon, so the waterfalls have to be contained in a spout or channel, and they can't be more than a centimeter high. When these conditions are met, the main flow of water through the center of the passage creates a backwards flow along the edges. Tea particles get caught in this flow and are washed into the higher container. This raises questions about the safety of chemical filtering systems, as well as the cleanliness of teapots.
Image: Patrick George