Here's why:
* Phoneme: A phoneme is the smallest unit of sound that can distinguish meaning in a language. Think of the difference between "cat" and "bat" - the sounds /k/ and /b/ are phonemes because they change the meaning of the word.
* Morpheme: While morphemes are smaller than words, they are larger than phonemes. A morpheme is the smallest unit of meaning in a language. For example, the word "unbreakable" has three morphemes: "un-", "break", and "-able".
* Word: Words are groups of morphemes that function as a single unit.
* Sentence: Sentences are groups of words that express a complete thought.
So, even though morphemes are smaller units than words, phonemes are the smallest individual sounds that change the meaning of words.