One spring evening in 2024, a science journalist named Rachel Gross bombed at karaoke. The culprit was a bleed in a fist-sized clump of neurons tucked down in the back of her brain called the cerebellum. A couple weeks later, her doctors took a bit of it out, assuring her it was just helping her with motor coordination — she might be a bit clumsy for a while, but she’d still be herself. But afterwards, she didn't feel like herself. So she dove into the dusty basement of the brain (and brain science) to figure out why. What Rachel found was...