Answer the following question given this paragraph: 

Digestion includes both mechanical and chemical processes. Mechanical digestion is a purely physical process that does not change the chemical nature of the food. Instead, it makes the food smaller to increase both surface area and mobility. It includes mastication, or chewing, as well as tongue movements that help break food into smaller bits and mix food with.


Q: Which form of digestion is a purely physical process that does not change the chemical nature of the food?


A:
mechanical