How to use your brain at 100%
TLDR: Please don't.
Studies have been conducted where subjects solved various tasks while their brain activity was observed via fMRI. If a subject had encountered such a task before, their brain activity was lower than that of a subject solving a similar problem for the first time. The second person's brain was straining and activating various zones, while the first one used resources economically and efficiently.
When we solve a familiar task, the brain works economically, like an experienced chef who easily handles cooking a dish. A novice, however, will rush around the kitchen, activating all their resources. Thus, it makes sense to become an expert rather than trying to set your brain on fire.
There is one state in which 100% of neurons are actually activated - an epileptic seizure. The brain works optimally by activating only the necessary neural networks for a specific task, conserving energy and preventing overload.
There's also a myth that our brain only works at 10%. Modern research methods show that even at rest or during sleep, almost all areas of the brain are active. Different tasks require greater activation of different areas, but other departments always get involved too.
Our brain weighs 2% of our body mass but consumes 20% of our energy. And evolution is very economical and calculating. So it would have long ago cut out those parts of the brain that aren't being used.
So our brain is an amazing smarty-pants - it's better to become its advanced user than to try to activate it at full capacity. Simple tips will help: learn new things, take care of your health, give yourself rest, develop social connections, and set yourself pleasant challenges.