I grow them by taking the brain cells from a mouse embryo. The brain gets mashed up so that all the cells are separated. Then you put them in a little dish with special liquid food and lots of heat. They start of as just little circles but after a few days they get bigger and sprout out long arms called axons and dendrties. These long arms allow the cells to signal to eachother and they form a spiderweb like network in the dish, like a very basic brain!