What are these 7,000 pairs of shoes doing in front of the U.S. Capitol? Avaaz, a New York-based human rights group, placed them there to represent the American children killed by guns since the 2012 Sandy Hook Elementary School shooting in Newtown, Connecticut. The makeshift memorial appeared for several hours on March 13—less than a month after 17 people were gunned down at Marjory Stoneman Douglas High School in Parkland, Florida.

Avaaz collected the shoes from donors around the country, including victims’ families. Volunteers arranged them—one pair for each of the approximately 7,000 children lost to gun violence since Newtown—on the Capitol lawn while a U.S. Senate hearing on gun control was going on inside.

“This project is about putting the human cost in front of lawmakers,” says Emma Ruby-Sachs, deputy director of Avaaz and the organizer of the project. “It’s a monument to the kids whose voices were silenced and recognition that kids are leading this movement now, and it’s time for adults to get on board.”