The phrase "Old and New" is repeated throughout The Clouds in order to advance and emphasize the thematic issue of the proper educational system: old (meaning, traditional) or new (meaning sophistic). The day of "Old and New" is the Athenian term for the end of the month. Since months were calculated according to a lunar calendar, the end of the month occurred with each cycle of the moon: the last day of the "old" cycle of the moon and the first day of the "new" cycle of the moon. Debts were collected on this final day, the day of "Old and New," and it is Strepsiades's anxiety over this day of (financial) reckoning that leads him to the wild and crazy notion of learning sophistry in order to outwit his creditors in court.
Currency, or money, is central to this play: the wealth discrepancy between Strepsiades and his wife makes his marriage uncomfortable and it is his debts that make his nights unbearable with worry. The notion of money suggests a legacy and the notion of a legacy or inheritance of money suggests familial relationships. Therefore, the idea of money and family are intertwined.
Money and the notion of debt are also important to this play in terms of its tragic arc: in tragedy, the tragic hero "pays for" his tragic flaw by his suffering. Thus, in this play, Strepsiades receives back, at Pheidippides's pummeling hand, the bruises he has dealt out to slaves, students, and creditors, with added interest as well. The notion of "revenge" (II.i.1506) itself contains an awareness of finance: giving someone what is due. Aristophanes's tragi-comedy is driven by the question of "what is due" in particular: Strepsiades owes his creditors money, and when Strepsiades misbehaves by deciding not to pay this debt, he pays for his poor choice in other ways.
The word "currency," in particular, carries with it a potent pun on the word "current." As mentioned above, much of this play is concerned with the question of how to digest much of the new or "current" information—be it science, sophistry, or atheism—that was circulating in fifth-century Athens.
Much of the humor in this play derives from Aristophanes's hysterical invention of specifically gendered nouns, such as "chicker," "chickeness," "trough," and "triffen," (I.ii.658). Not only does Strepsiades flaunt his newly enriched vocabulary to his son Pheidippides, but he also uses it as the basis of his abuse of his two creditors. The words that Aristophanes invents are delightfully droll and because of their novelty, they seem to us and to Strepsiades to represent the heights of chic. Why else would Strepsiades waste so much time and breath clucking over "chicken"?
However, the words that Socrates teaches Strepsiades are precisely as banal as that: chicken, trough. He does not teach Strepsiades the proper names for potions, spells, or distant kings. Rather, he focuses on the trivial, boring vocabulary. Therefore, in spite of how delicate "triffen" may sound, the audience must realize that pigs eat from it! The disjunction between the prettiness and level of specialization between the invented word and its representative object creates great comedy. This disjunction also reemphasizes the key theme of the unfortunate disconnect between intellectualism and pragmatism, grand knowledge and the real world.