Most interjections are not onomatopoeia. An interjection is a word that expresses emotion or exclamation, such as wow!, eek!, and d'oh!. It usually is not part of a sentence. Onomatopoeia is a word or phrase that imitates the sound of a thing or action, such as splash!, badaboom, and whoosh. It can express emotion, but the imitation of sound is more prominent. In the sentence 'Hey, get off my lawn!', the interjection Hey does not imitate the sound of anything other than itself, the person saying 'hey', and in this case it is an exclamation, but maybe it also expresses some emotion (anger). In the sentence 'Achoo! I think I caught a cold', the onomatopoeia Achoo primarily serves to imitates the sound of sneezing.
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