In the movie “Lucy,” opening Friday, Scarlett Johansson’s title character develops superhuman powers when she becomes able to use more than the “normally allotted 10 percent of her brain.”

The only problem is that we actually use 100 percent of our brains already. It’s not clear how the myth originally started. Some think it was an Albert Einstein misquote, or this 1908 statement from William James: “We are making use of only a small part of our possible mental and physical resources.” But we’ve known for some time that it’s not true.

Over the course of a day, humans tend to use 100 percent of their brains, John Henley, a neurologist at the Mayo Clinic in Rochester, Minn., told Scientific American.

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In the movie version, though, Morgan Freeman — who plays the all-knowing scientist — spreads the 10 percent myth. When Lucy is made into a drug mule and a bag of a new street drug that’s been sewn into her stomach ruptures, she finds she can use more than 10 percent of her brain. Her character is transformed.

Even when our brains get damaged, it’s often to a tiny part of the brain. A stroke, for example, affects a small part of the brain, but can cause life-altering disabilities.

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And when we’re asleep, our brains remain highly active, accessing areas of the brain you might think would be dormant, such as the higher-level thinking areas such as the frontal cortex.

In the end, writes Robynne Boyd in Scientific American, although the 10 percent doesn’t apply to how much we use our brains, it may apply to how much we understand how our brains function.

Photo: Universal