The character 脑子 is formed by combining the character 脑, which pictographically represents the brain (derived from a radical meaning "flesh" or "body part" combined with a component suggesting the convoluted shape of the brain), with the character 子, a common suffix that, in this context, acts as a nominalizer to create a colloquial, everyday noun. Together, they build a single word where the first part provides the core meaning and the second part completes it as a familiar, concrete term.