How all the thousands of different types of cells in the adult human body—ranging from brain to blood to bone—arise from a single cell during embryonic development is a biological marvel and mystery. Childhood cancers are thought to arise from defects in how cells and organs form during embryonic development. However, the precise developmental origins of childhood cancers remain largely unknown because it is not possible to study how human embryos naturally develop in the womb due to technical and ethical limitations. We must find alternative approaches to study the “black box” of human embryonic development if we are to understand how, when, and why childhood cancers emerge. Starting from embryonic stem cells, we are reconstructing the development of different types of human cells in a Petri dish, thus providing a tractable and modular platform to examine how defects in developmental processes can lead to childhood cancers. As a stem cell and developmental biologist, I am indebted to Stanford’s collegial and interdisciplinary environment, which has led to multiple collaborations with other Stanford Ludwig Center investigators to make this work a reality. Research by my laboratory has led to various awards, including the NIH Director’s Early Independence Award, and I have been selected to the Forbes 30 Under 30 and have been named a Packard Foundation Fellow, Pew Scholar, Human Frontier Science Program Young Investigator, Baxter Foundation Faculty Scholar and The Anthony DiGenova Endowed Faculty Scholar.