"Have you ever really looked at your hand? Like, with 100% of your brain?"

The absolutely idiotic idea that we only use 10% of our brains comes from the fact that roughly 10% of our brain matter is made up of actual grey matter. This is like saying you only use 10% of your car because most of the car is not the engine. It's stupid, and people who write entire movies around this concept (I'm looking at you, Powder and Limitless) are stupid, but they think that you are even stupider. To quickly sum Lucy up, Scarlett Johansson has her brain improved by science, which leads to her becoming magical. Like, Matrix magical.