Zebrafish aren’t just surrounded by liquid, but turn liquid—in part—during their development. As the zebrafish embryo develops from a ball of cells to a fully-formed fish, a region of the embryo switches its phase from viscous to liquid in a process known as fluidity transition. Such fluidity transition has long been speculated to exist in living matter, but is described for the first time to occur in a living organism in a study published today in Nature Cell Biology. The study was carried out by the group of Carl-Philipp Heisenberg at the Institute of Science and Technology Austria, with first author and Postdoc Nicoletta Petridou, together with the groups of Guillaume Salbreux at The Francis Crick Institute and Edouard Hannezo, also at IST Austria.