Theory:

Personification is a figure of speech, a poetic device in which human attributes are given to animals, non-living things or ideas. The objects speak/behave like humans, with human emotions in such cases.
Example:
Personifying animals:
  1. The cat asked the mouse for a dinner treat.
  2. The dog asked his master to let him inside.
Personifying ideas:
  1. Love is blind.
  2. His anger oozed out in his words.
Personifying non-living things:
  1. The popcorn popped out of the bowl immediately in the oven.
  2. The sun shined and smiled upon us, early in the morning.
Uses of personification:
  1. When human qualities are given to objects, the writer/poet can convey the meaning easily.
  2. The understanding of the idea is made easier when objects are personified.
  3. It helps to build a character around the object.
  4. Makes the text/poetry more interesting and fun to read.
  5. Literal meanings are to be ignored in such cases.
Personification used in the poem "The Wonderful Words":
  1. Never let a thought shrivel and die - In this line, thought which is an abstract non living thing, is considered as a human, since it says thoughts may die. Whereas thoughts do not have life, and cannot die. It adds effect to the poem.
  2. To the brightest thoughts...… And handsomely groomed and fed— Again, thoughts are given human qualities, as it says thoughts can be groomed and fed. Only humans can groom and feed themselves. It is used to refer that once the words are used, thoughts become clear.
  3. They give it its body and swingThoughts are said to be having a body in this line. The personification is used, to state that words can give a form to the thoughts just like body gives shape to humans.
  4. But only words can free a thought/From its prison behind your eyes - Both freeing and being a prisoner is a human activity. Here thoughts are compared to a human being in a prison, and words as the power that frees it from the mind.