Here's why:
* It's a distinct sound: It's different from other sounds like "p" or "g".
* It changes the meaning of a word: Compare "cat" and "cot" - changing the initial "k" to a "c" changes the meaning of the word.
* It's the smallest unit of sound: You can't break down "k" into smaller meaningful sounds.
Other examples of phonemes include:
* Vowels: /a/ (as in "cat"), /i/ (as in "see"), /u/ (as in "book")
* Consonants: /b/, /d/, /f/, /m/, /n/, /r/, /s/, /t/, /v/, /w/, /z/
Keep in mind that the number of phonemes can vary depending on the language. For example, English has about 44 phonemes, while some other languages have more or less.