Fluids are described as materials that do not retain their shape, but take the shape of their container. Fluid foods include liquids such as water and oil, suspensions such as some fruit juices and purees, emulsions such as mayonnaise and ice cream, and dispersions such as those made with gums and starches along with the gels that form after they are heated. Fluid foods are also frequently heterogeneous mixtures of water, fat, proteins, and carbohydrates, which give them very complex behavior. The movement of these fluids must be controlled during unit operations such as pipe flow, pumping, pasteurization, mixing, filling, metering, etc. This provides the degree of processing and the inputs required by heat and mass transfer, as discussed in chapters 10 and 11. The principles used to predict how fluids will behave in a given system are referred to as fluid mechanics.