Questions that require continuous single-cell observations to untangle alternative interpretations. (A) Snapshot analysis is not able to discern between instructive and permissive roles of cytokines during lineage decision. The selective model requires cell deaths prior to terminal differentiation. Proof for the absence of cell death requires knowledge of the fates of every single cell over time. Using this approach, it was demonstrated that certain cytokines can instruct lineage choice. (B) The presence of blood cells on top of endothelium can be explained by generation of blood cells from hemogenic endothelium or from alternative sources. The direct observation of endothelial to hematopoietic transition using continuous observation provided direct evidence for the existence of hemogenic endothelium. (C) Lineage decisions based on the stochastic fluctuations of 2 antagonistic lineage promoting transcription factors require a (transient) double-positive cell state. By demonstrating the absence of the PU.1/GATA-1 double-positive cell state during most differentiations into the myeloid or megakaryocytic-erythroid lineage, this model could be refuted as the central mechanism of myeloid lineage choice. (D) The acquisition of different daughter cell fates can be regulated by mechanisms acting after or during cell division. Untangling both mechanisms requires continuous quantitative single-cell observation and the demonstration that different daughter cell fates can be predicted by the asymmetric inheritance of factors during mitosis.