Second-order conditioning involves the pairing of two CSs (CSI + CS2), rather than the pairing of a CS and US (CS + US). Pavlov (1927/1960) conducted experiments that demonstrated second-order conditioning. The tick of a metronome was paired with food. The sound of the metronome came to evoke salivation. Once the ticking sound reliably elicited salivation, Pavlov paired it with the sight of a black square (CS, + CS2). Following several pairings of the metronome beat with the black square, the sight of the black square evoked salivation.