The teacher gives the first line and asks students to compose the matching second line.
Tips
culture
对对子 is a classic Chinese literary game: one person gives the 上联 (opening line) and another must produce a 下联 (response line) matching in length, tone pattern, and parallel grammar. It was a standard part of imperial-era schooling.
memory
Three characters that look almost identical: 对 (to match) + 对 (couplet) + 子 (suffix). The first 对 is the verb 'to match', the second is the noun 'couplet'.