First, they created a 3D-printed gel model of a developing fetus’s smooth brain, before the folding happened. The model consisted of two gel layers; an inner layer to mimic the white matter and then an outer elastomer gel to coat the inner layer, acting as the outer cortical grey matter. Together, the two layers mimic brain growth. After they placed the model in the solution, the fetal gel brain quickly formed into the characteristic folds, just as the researchers suspected. As the different layers of gel expanded, even slightly, they generated compression forces that forced the outer layer to draw back.