Most of us have long accepted that our brains look like overgrown, shriveled walnuts. But why do our brains have those telltale wrinkles?

The cortex, or the outer surface of the brain — what’s colloquially referred to as “gray matter” — expands and subsequently folds as our brains develop in the womb, said Lisa Ronan, a research fellow in the Department of Psychiatry at the University of Cambridge in England.

In essence, this expansion causes pressure to increase in that outer surface, which is then mitigated by folding, Ronan, told Live Science. [What If Humans Were Twice as Intelligent?]