Ketchup, in addition to being an exceptionally popular condiment, is also a non-Newtonian fluid. This is the key to its curious behavior. As a non-Newtonian fluid, ketchup’s viscosity — and consequently its ability to flow — varies with the velocity gradient across the fluid or the shear rate. When you first turn the ketchup bottle upside down, it’s probable that you will get only very minimal flow, if any at all, due to its high initial viscosity. Fortunately, ketchup also has an interesting shear thinning property. When an external force is applied in addition to gravity, the increased shear will result in decreasing the ketchup’s viscosity, allowing the product to flow more readily. This is why we sometimes must tap or shake the bottle to enjoy ketchup with our fries.