We only use 10% of our brain at a time. Because using 100% of your brain is called a seizure.

Source: Once used 100% of my brain.

Yep. The rebuttal that stuck with me was "We only use 33% of traffic lights."

Tbf, they might have been right about themselves at the time they thought that.

Even that is a common misconception. Our brains are working fully, all the time.