In the weeks leading up to Open Source Gallery’s annual Soap Box Derby on Saturday, kids ages 7 to 12 have worked with artists to build soap box racers out of found materials. You can see which child fashioned the winning contraption during the kids race at 11am, and then see what the big kids have built during the adult race at 1pm on 17th St. between 5th and 6th Aves. This year, more adults than ever are expected to race since the gallery has been hosting soap box-building workshops on Thursday and Friday nights from 6 to 9pm since late July. The last two are this week, and with a $40 donation, you can still build a car using Open Source’s tools and materials, except for the wheels, which are BYO. (Call ahead to reserve a spot: 646-279-3969.) In fact, if you have any stroller, skateboard, roller blade or kids bike wheels to spare, drop them by the gallery between 9am and 3pm this week.