Grown from pluripotent, adult human stem cells, organoids offer just such a model. The stem cells can be generated from skin, blood, or other cells, then coaxed into developing into cells from a body part of interest. Different types of organoids can be combined into "assembloids" to better model complex organs that natively originate from combinations of different tissues. But a living, functioning heart is more than a cluster of certain types of cells. To create a model, Li grew a valve on the surface of a heart assembloid— two organoids made from different types of heart cells that were combined into one platform.