We feel we understand how things work because we can't tell the knowledge in our own heads apart from the knowledge we can reach in other people, tools and our surroundings. Since we treat what the community knows as if it were our own, we live believing we understand far more than we do. The problem isn't that people are ignorant, but that they are more ignorant than they think.
People who rate their grasp of an everyday object like a zipper as strong find they have almost nothing to say once asked to explain, step by step, how it works. After trying, they rate their own understanding lower. Being asked for a detailed causal explanation is what exposes the gap between feeling you know and actually knowing.
Because we store most knowledge in others and in our environment, what sits in any one head is mostly superficial. This is as much a strength as a flaw: it lets societies build complex technology while most people can't explain a pen or a toilet. In exchange, each person today knows far less hands-on than a Stone Age forager who could make clothes, start fires and hunt alone.
Intelligence lives less in individual brains than in the community of people, tools and surroundings we constantly lean on without noticing. Individuals know little, yet a group dividing the labor of knowing can fly planes and reach the moon, feats no single person fully understands. Seen this way, a person's intelligence is best measured by how much they add to a group's reasoning and problem-solving.
Because we borrow our sense of understanding from the people around us, we hold firm views on issues we couldn't actually explain. That is why political opinions and false beliefs are so hard to change, and why the illusion feeds political gridlock. More reflective people show less of this illusion of understanding.