Pesah - Shir Ha'Shirim
On Shabbos Hol Ha'Moed of Pesah we have a custom to read Shir Ha'Shirim.
In 1:1 of Shir Ha'Shirim, the passuk starts off saying, "שיר השירים אשר לשלמה". Rashi tells us that Shir Ha'Shirim is an especially holy song and book, much more so than any of the other כתובים - later biblical writings. Rashi compares it to a king who gives a kilo of wheat to a baker and tells him to sift out the various levels of impurities and use the flour to make a special delicate loaf of bread.
So too, Rashi says, is Shir Ha'Shirim. All the כתובים are holy, but Shir Ha'Shirim is קדש קדשים - especially holy.
Kodesh Kodoshim is not a separate entity, in the sense that you have two levels; holy and holy of holies, with them being independant of each other. You achieve the level of "especially holy" by taking the holy and continuing to refine it.
You can have "holiness" and still not be completely pure. If you take that "holy" and continue to sift out the impurities you can then achieve the ultimate level of holiness, holy of holies.
This is similar to Hametz itself and its disposal and metaphorically to our actions. If we continue to sift out the hametz from within us we can continue to improve our ways and strive and even achieve higher levels of kedusha, eventually reaching the status of holy of holies.