A Brooklyn elementary school was transformed into a high-tech laboratory during a Cornell-led science discovery day Oct. 4.

About 300 schoolchildren jammed all corners of Public School 21 as part of the 10th annual 4-H National Youth Science Day, which reached an estimated 100,000 schoolchildren in 50 states.

This year’s interactive learning challenge, Incredible Wearables, was developed by the University of Nebraska- Lincoln. Youths engineered and built electronic armbands that measured their fitness while exercising.

The PS 21 gymnasium was filled with “wired-up” students jumping rope, spinning hula hoops or running in place. Fellow youth scientists then monitored and measured heartbeats and number of steps or jumps taken.

In another room sponsored by faculty, staff, and volunteers from Cornell University Cooperative Extension-New York City (CUCE-NYC) and National 4-H Council, the school children explored New York state’s parks using a giant geological map; learned about hydroponics, or growing plants without soil; learned about energy by pedaling a bicycle that produced electricity to power light bulbs and a fan; created bird feeders from pine cones and planted fall bulbs to help pollinators; and learned about sugar levels in juices, beverages and fatty foods.

The goal of the day was to introduce youth to science, technology, engineering, and mathematics and encourage them to pursue careers in those fields.

Another interactive exhibit, sponsored by Google, allowed students to wear virtual reality goggles that exposed them to moving undersea images, a dairy farm in Minnesota and other science-based scenes.

Lucinda Randolph-Benjamin, CUCE-NYC extension associate for 4-H youth development, said this year’s combination of high-tech fitness tests and interactive exhibits transformed the flagship Brooklyn school into a “crazy but incredible learning environment.”