With dozens of followers on Sina Weibo, Zhang posted a call for participants on the social media platform in 2012. Opening a small photo studio in Wudaoying Hutong, Beijing's Dongcheng district, he handed out handbills to introduce the project to passersby.

It had three questions. Who are you? When do you want your photo to be exhibited? What moment (with who and where) do you want to capture?

Most people were skeptical and asked Zhang questions like what would happen to the project if he died.

This led him to question the meaning of what he was doing. His answer came in the form of a woman in her 80s he met on the street. After hearing about his project, she told him with a glint of curiosity in her eyes that she wanted to be photographed and to see herself a decade later.

"I still remember that moment. The silver-haired lady didn't question whether she would (live to) see the photo or not. It was the answer I had been looking for," Zhang says with a smile.