Brain balls sound like something straight out of a Tim Burton movie: starting as stem cells harvested from patients, they eventually develop into masses of living neurons, jumbled together in misshapen blobs.

Just like the developing brain, these neurons stretch and grow, reaching out skinny branches that grab onto others to form synapses—junctions where one neuron talks with the next.

And they do talk: previous attempts at growing these “brain organoids” found that they spark with electrical activity, much like the webs of neurons inside our heads that lead to thoughts and memories.