Mao Weiyu is now in her 80s. Despite shooting strangers commuting underground, her choice of those to photograph is not random. She only snaps pictures of those who offer their seats to her as a thank you gesture. /Tencent Photo

Mao started this habit in 2015, using her tablet for selfies. /Tencent Photo

She has collected pictures with over 57 strangers she met by pure luck on the subway. /Tencent Photo