In a junkyard just off 19th Street in Park Slope, 8- to 14-year-olds have been sawing, drilling and painting, surrounded by brightly colored piles of wood, rope-and-tire swings and lots of wheels.

They’re getting ready for Saturday’s 18th annual South Slope Derby, when hundreds of spectators will cheer on dozens of kids racing their own custom-built cars down 17th Street. Every summer, KoKo NYC — the program that runs the race — organizes workshops where kids design and build their vehicles, applying the nonprofit’s “trash is treasure” mission of teaching kids to use their creativity and reclaimed materials to build anything possible.

“It really puts engineering skills to the test, and I want to be an engineer when I grow up,” 8-year-old Bowie said as he and his fellow 8-year-old partn

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