The relative contribution of physical and chemical factors in organogenesis in general and in cardiogenesis in particular is an old and still unsolved question in developmental biology (Chang 1932, Anatomical Record, 51, 253-265). A recent Nature paper (Hove et al. 2003, Nature, 421, 172-177) strongly suggests (but does not prove) the important role of blood flow and associated shear stress in cushion tissue morphogenesis. However, another elegantly designed study (Bartman et al. 2004, Public Library of Science-Biology, 2, E129) raises questions as to the validity of the shear stress hypothesis and the role of physical factors in cardiogenesis. Although these studies advance our understanding of cardiac development, other possible explanations for the role of blood flow in cardiac organogenesis remain to be addressed.