Functional foods are foods that have had ingredients specially added to give the food an added function, such as an added health benefit. Adding vitamin D to milk is an example. Adding live bacterial cultures to yogurt is also an example. This last example is also an example of a probiotic. A probiotic is a food that has live bacterial microorganisms present in the food that are beneficial to people. These are common in fermented foods. A prebiotic is a non-digestible additive to food that is meant to enhance the activity of beneficial bacteria in the digestive system.