When neuroscientist Madeline Lancaster was a brand new postdoc, she accidentally used an expired protein gel in a lab experiment and noticed something weird. The stem cells she was trying to grow in a dish were self-assembling. The result? Madeline was the first person ever to grow what she called a “cerebral organoid,” a tiny, 3D version of a human brain the size of a peppercorn. In about a decade, these mini human brain balls were everywhere. They were revealing bombshell secrets about how our brains develop in the womb, helping treat advanced cancer patients, being implanted into...