I think one of the major problems facing the flat earth movement is that it attracts certain kinds of people who for whatever reason are likely to accept things like "water always finds its level" as evidence against the globe at face value. I have talked to flat earthers who are clearly very intelligent but who will still use arguments like water finding its level and others such as the idea that a gas cannot exist next to a vacuum without a container. These two arguments are similar in that they are effectively meaningless. They are really arguments against the existence of gravity, since if you assume gravity, gas can exist in a pressure gradient next to a vacuum, and water will find its level around a ball. These flat earthers are in many cases perfectly capable of making arguments that are more reasonable, but for some reason, they do not. Instead of debunking the Cavendish experiment to disprove gravity, they simply state that water always finds its level and proceed to ask for a demonstration showing otherwise. I'm not sure what the source of this tendency is, but it is MUCH harder to debate a flat earther who uses arguments that actually (kind of) hold water.
