They were challenged to make a balloon hover over one of the blue plants on a makeshift landscape. They could use background knowlege about gravity and air currents to make the balloon craft move to the correct location and hover.
Research shows that students respond better to STEM challenges when there is a story connected. The students were told that the landscape was a rainforest and that the blue plants had a rare medicinal plant that needed to be harvest. They had to hover over it to harvest it.
For more STEM challenges for Elementary see STEM Engineering Challenges for Elementary Education