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.
These Elementary Education preservice teachers had a great time! 
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.
NGSS STEM Engineering Challenges 3-5-ETS1-1, 3-5-ETS1-2 and 3-5-ETS1-3
For more STEM challenges for Elementary see STEM Engineering Challenges for Elementary Education
