Hello Thank you , Some nouns could be both uncountable and countable such as the word "hair " Her hair is blond. ( here it is uncountable) There is a hair in your tea .( here it is countable) 一根头发 = a hair ( here it is equivalent to the countable word "hair" in English) 一根胡子 = a beard

Hair is countable, water is uncountable. However, glass of water, it is countable, it counts the glass, it isn't count the water. 一根头发, Is it a hair ? It is grammatically correct. 一 =one, 根=piece, 头发=hair . According to your Chinese writting in a question form, it should be written as: is it a piece of hair?