A membrane has a positive curvature when it bends towards the inner or cytoplasmic side. In contrast, a membrane has a negative curvature if it bends towards the outer or extracellular side. There are multiple mechanisms that can cause membrane bending. Lipid-induced membrane bending is due to the asymmetric aggregation of specific lipids in the membrane. Lipids with large head groups induce positive membrane curvature, whereas lipids with small head groups produce a negative membrane curvature. Protein scaffolding is another mechanism where proteins either deform the membrane or stabilize an already deformed membrane. In a different mechanism, hydrophobic protein domains are reversibly inserted in one layer of the membrane to increase its area compared to the other layer. This asymmetry forces the membrane to bend. Finally, the push and pull forces generated by the assembly and disassembly of the cytoskeleton and its associated motor proteins can also bend the membrane to form projections, such as filopodia in epithelial cells.