Imitation is a music compositional technique that creates polyphony by having each voice enter in a similar manner but then continuing in different directions. Imitation is like a conversation. Each voice needs to have similar material to make the conversation work. A verbal metaphor would be one statement What color is the sky? with an answer, The sky is blue. If the response was The grass is green. the conversation would not work. But, since the sky was used in both statements then the conversation makes sense. Imitation in music functions the same way.