Chocolatiers have been perfecting the art of the bonbon since the 1600s, passing down techniques for crafting a smooth chocolaty shell. Now, a theory and a simple fabrication technique derived by MIT engineers may help chocolate artisans create uniformly smooth shells. The research should have uses far beyond the chocolate shop - by knowing just a few key variables, engineers could predict the mechanical response of many other types of shells, from small pharmaceutical capsules to large airplane and rocket bodies. The fabrication technique involved drizzling liquid polymer over dome-shaped molds and spheres such as ping pong balls. The researchers allowed the liquid to coat each mold and cure over 15 minutes. They then peeled the resulting shell off the mold and observed that it was smooth, with a nearly uniform thickness throughout. They developed a simple formula to estimate the final thickness of a shell, which essentially equals the square root of the fluid’s viscosity, times the mold’s radius, divided by the curing time of the polymer, times the polymer’s density and the acceleration of gravity as the polymer flows down the mold. The formula boils down to the following relationships: The larger a mold’s radius, the longer it takes for fluid to flow to the bottom, resulting in a thicker shell; the longer the curing time, the faster the fluid will drain to the bottom, creating a thinner shell.