Synecdoche and metanomy have which thing common or different?

In synecdoche a part of some object is allowed to represent the whole object.

“He has many mouths to feed.”

Mouths ,here,are representatives of human bodies.This connotes that he has many people to feed or he manages the food expences of a large family.Looking in a broader sense the same sentence does also showcase metanomic taste where in ,when two things are corelated we,some times, name one to mean another.

Eg.water flowing in a river is dubbed as the river is flowing.

Here the relation of the given objects is that of “the container and the contained”.Since the body taken as container and the mouth contained suffices the need.

The second sentence “the kettle is boiling” is a pure metanomy.Here the kettle is boiling connotes that it is actually the tea in the kettle which is boiling.

These poetic devices have good currency in our daily conversations.