During preimplantation development, the mammalian embryo forms the blastocyst consisting of a surface epithelium enveloping a fluid-filled lumen and a cluster of pluripotent stem cells. The architecture of the blastocyst is key to uterine implantation and further development. The shaping of the blastocyst is the result of changes in the physical properties of the cells, which generate the forces sculpting the embryo. These forces typically originate from the cytoskeleton, adhesion molecules of the osmotic machinery. In the lab, we use live microscopy to observe shape changes across spatiotemporal scales, biophysical tools to characterize the changes in physical properties of cells and tissues, and genetics to disrupt morphogenesis. This led us to discover the major contribution of cell contractility throughout preimplantation morphogenesis in both mouse and human embryos as well as the physical nature of the mechanisms leading to the positioning of the lumen within the embryo.