How human embryos can develop into identical twins - who share the same DNA - is largely unknown. We know that identical twins come from a single fertilized egg that splits into two and it is believed that the timing of this division affects whether the twins will share a placenta and/or an amniotic sac. However, what exactly happens and when was not clear due to the rare incidence of monozygotic twins (around 0.4% of pregnancies) and the impossibility to study this in the uterus. We have now succeeded in forming embryo-like structures of human identical twins purely from stem cells. This advancement makes it possible, for the first time, to glimpse into the processes of how identical twins form. Key to this process is the rapid expansion and subsequent division of the blastocyst, a structure comprising future placental cells that encapsulates the stem cells destined to develop into the embryo.: accelerated expansion of the blastocyst—the sac of future placenta cells that encases the stem cells from which new life can emerge — splits the embryo into two. This discovery has been published in Advanced Materials (Dorian Luijkx et al. https://doi.org/10.1002/adma.202313306 )