Surely you've heard someone say that humans only use 10% of our brains (and some people even less), but that turns out to be a just myth:
William James, a psychologist in the 1800s, once metaphorically used the idea of 10% of the brain being all that was used at one time. This grew into the rumor that it was all the brain was overall and most of the rest was not understood or used as far as we know. Actually, the inactive neurons are just as important at any given moment as the ones actively firing at a point in time, and the 10% comes from varying areas at different times.
Read more human body myths at Environmental Graffiti: Link
Here's a quote from the introduction to Uri Geller's book Uri Geller's Mind-Power Book:
“Our minds are capable of remarkable, incredible feats, yet we don't use them to their full capacity. In fact, most of us only use about 10 per cent of our brains, if that. The other 90 per cent is full of untapped potential and undiscovered abilities, which means our minds are only operating in a very limited way instead of at full stretch. I believe that we once had full power over our minds. We had to, in order to survive, but as our world has become more sophisticated and complex we have forgotten many of the abilities we once had”
Another pop psychology myth that is even more widely believed is that people can be categorised as being either "left or right brain dominant".
The myth is that hair grows back thicker and darker after repeatedly shaving it, more so than if you were never to have shaved it at all. As an example, a woman who has shaved her legs for years, should she suddenly allow it to grow out, would have much thicker leg hair than a woman who never shaved her legs, according to the myth.
The truth is, however, that shaving does not make hair thicker. The fact that it's untapered when first growing out makes it feel and appear coarser, which contributes to this myth. More so, however, is the fact that most people's hair tends to get thicker as they age, anyway, so suddenly stopping shaving after years reveals hair that would have been thicker, anyway.
This myth is why former competitive swimmers always contribute their thick body hair to having shaved it all off for years, beginning when they were young. But that fact is it would've become that thick, anyway, had they never shaved.
Another thing that's not true is that kids "burn out" after eating sugary food. OK so maybe the energy peak is higher and quicker from eating sugary food than from eating other more "healthy" food. However they will end up back at the same baseline whatever they have eaten. Sugar doesn't mysteriously sap their energy. That Sportacus has a lot to answer for.