We are using a simple model, the nematode C. elegans, to analyze how mechanical forces influence specific cellular processes and conversely on how tissues withstand forces during morphogenesis. The C. elegans embryos extend by a four-fold factor along their anterior-posterior axis. Two main active mechanical forces power this extension: epidermis actomyosin contractility and muscle contractions, which exert mechanical forces on the outer epithelium. I will discuss how these forces can polarize the anterior-posterior extension by three factors, stress anisotropy, stiffness anisotropy, polarized trafficking.