One of biology's great mysteries is how a single fertilized egg gives rise to the multitude of cell types, tissues, and organs that fit together to make an adult body. Now, a combination of single-cell sequencing technologies and computational tools is providing the most detailed picture yet of this process. Researchers made multiple snapshots of gene activity in most of the cells in developing zebrafish or frog embryos. They then assembled those data, taken at intervals of just minutes to hours, into coherent, cell-by-cell histories of how those embryos take shape. The analyses led to some surprises, including that cells develop cell type-specific gene activity profiles early on, and yet, later, some cells ultimately stray from their developmental trajectories. Such studies could also provide a recipe book for stem cell scientists and tissue engineers who want to make various cell types.