Baptista compares his backing of this shaky marriage between Katherina and Petruchio to a merchant entering into a risky business venture. Tranio continues the metaphor by comparing the unwed Katherina to a perishable commodity whose value declines with time, and says that if this marriage doesn’t work out, Katherina will be as worthless as a ship’s cargo lost at sea. Tranio’s also plays on the word “fretting” — Katherina is fretting (slowly wearing away), as well as fretting her father (worrying him).