The term "BOGO" ("Buy One Get One", also referred to as "Buy One Get One Free") has entered the urban dictionary, and is a sales tactic whereby sellers offer buyers a second unit of the item free if they pay full price for the first unit. A variation of BOGO occurs when stores offer a "Buy One Get One Half Off" sales promotion. Why do you think consumers respond to the "Buy One Get One Half Off" sales promotion and what principle of economics does this behavior reflect?