how long is a piece of string
It's interesting, because the length of any specific piece of string is directly measurable, whereas in the general case it's indeterminate because of the wide range of non-standard lengths of pieces of string. This (generality) is a crucial feature of the question How long is a piece of string?. If it were an enquiry about the specific length of a definite section of twine, then it would be readily answerable. The distance between one end of a string and the other is clearly measurable. The cord has a definite length.