A:Too hot! This summer's weather is abnormal.
B:It's a pity that our classroom is not air-conditioned.
A:Air-condition? Are you dreaming? We even haven't a fan.
B:Maybe a book can do duty for a fan.
A:Yeah, We are so poor.
do duty for something ：充当，代替......之用
e.g. Let’s roll the log up to the fire; it can do duty for a bench.
do duty for someone：替某人履行职务
E.g. Mr Rogers will do duty for me while I'm away next week.
译：The classroom is not air-conditioned.