The 10% brain thing is now thought to be an urban legend and started by some people misinterpreting some scientific research on how brains work. Evidence that we use more than 10% of our brains includes studying people who have a small amount of brain damage caused by an accident or a stroke. If we only use 10% of our brains, the chances are a small amount of damage would be in the other 90%. It’s not the case and very small brain injuries can have a big effect on what people can do.
We can also use techniques called functional magnetic resonance imaging scans that show areas of the brain ‘lighting up’ when people think specific things, solve puzzles or do other things. This gives evidence that lots of areas of the brain are used even for simple tasks and disproves the 10% idea.
On top of that the brain is really useful and adaptive and even after people have had brain damage caused by strokes or other illnesses the brain can create new pathways and essentially rewire itself to be able to do the same as it did before using different areas of the brain.