I'm thinking like this: If the area of the cake is 1, then it's obvious that one third of the cake is (1/3), therefore two thirds is (2/3).

But it's a bit vague to me, why do the denominator and the numerator represent the whole and the part respectively? I mean, arithmetically, (2/3) means three divides two. From where does the part-to-whole interpretation of division comes from?