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…