Remember why a simple animal eats? If a cell inside the animal requires more chemicals from outside, it attracts outside chemicals through the service provider (Service provider is the light blue one in the middle. Please wait to see picture move).
Cells attract food through the service provider

The reason a cell must attract outside chemicals through the service provider is because that the service provider is the most conductive. Service provider automatically gets involved.

The brain is the most conductive service provider in an animal. It is basically formulated from some of the most conductive chemicals collected by the animal. In simple animals, the brains usually locate in the middle. As time goes by, animals became more complex, and the brains were relocated to other places due to many reasons: weather, chemical changes, environment, etc.

Since the brain is formed by some of the most conductive chemicals of an animal, one law were established.

There exist more than billion types of cell frequencies in one modern, complex animal. What is a frequency? A unique sound is a frequency. A request to attract a particular new outside chemical is a frequency. A particular smell is a frequency. A color is a frequency. A feeling is a frequency. Hearing is frequency. Etc.

An ice cream has its unique frequency....
ice cream
Are you hungry for this ice cream? Or not? If you are, a certain part of your body is requesting to attract chemicals from this ice cream. If you don't have the appetite, your body doesn't need any chemical from the ice cream. Your brain is most conductive and serves as the middleman.