Greater Twin Cities United Way is doubling down on Action Day, its annual celebration and volunteer event that helps needy families during back-to-school time.

Nearly 4,000 volunteers will stuff 25,000 backpacks — twice as many as last year — with school supplies on Aug. 9 at Target Center. The backpacks will be distributed to more than 50 Twin Cities nonprofits and school districts, which will in turn hand them out to kids.

All seven of Minnesota's pro sports teams have agreed to donate prizes and send some volunteers for Action Day: Game On For School.

"Action Day is one way we work to eliminate educational barriers, because we know when our kids do better, our community does better," said Trent Blain, interim president and CEO of Greater Twin Cities United Way.

More than 100,000 students in the Twin Cities go back to school without the supplies they need, according to the 2016 American Community Survey, conducted by the U.S. Census Bureau.

Classroom supplies and school activity fees cost an estimated $662 for an elementary school student and $1,489 for high schoolers, according to the 2017 Backpack Index computed by Huntington Bank and the nonprofit Communities in Schools.

The Timberwolves and Lynx are co-hosting this year's Action Day, which runs from 3 p.m. to 8 p.m.

Now in its 11th year, Action Day is meant to be a fun way to introduce children to volunteering, United Way leaders said. In addition to stuffing backpacks, the event includes games and a miniature Crayola Experience, where kids can color bookmarks and write messages for the kids who will be receiving the backpacks.

Several Twin Cities companies donated money to buy the supplies — enough to fill four semitrailer trucks.

United Way's long-term goal is to send every student in the greater Twin Cities area to school with a backpack full of supplies by 2021. People can donate or register to volunteer at gtcuw.org.