There are some myths out there which are just blatantly false, but that are really pervasive:
- Water in a sink on one hemisphere does not flow from the sink the other way than on the other hemisphere. There is such a thing as the Coriolis force, which (basically) describes the deflection in the path of an object moving on a rotating sphere due to its inertia. The Coriolis force does explain why cyclones rotate in the opposite direction on the northern hemisphere than on the southern hemisphere. But the force is simply way too small to have any effect on the rotation of water draining from a sink.
- Hair is not alive, it's dead. The only thing alive is the hair-sack beneath the skin, from which the hair grows. Same goes for nails, as both consist of keratin
- Split hair-ends cannot be mended. Once split, they stay split. Apparently, some products out there will glue them back together temporary, but after it wears off, the ends split again.
- Hair does not grow back thicker when you shave it. It just appears that way because the hair-stems are thicker than the ends.
- Your hair cannot turn gray all at once if something scares you.
- Going swimming after having eaten something will not give you cramps and make you drown.
I thought about citing references, but that would be useless. People who believe (and keep perpetuating) these myths are apparently hellbent on keeping those beliefs, no matter how much evidence you show them that it's false. For the others out there, google around a bit. You'll find enough information dispelling these myths.