There are a lot of STEM challenges out there. But C'mon people! Just giving your students a challenge and sitting back and watching them is not REALLY teaching. I would argue that It's NOT really "STEM" unless it follows the NGSS Eight Practices of science and engineering (here's the link: http://www.nextgenscience.org/sites/default/files/Appendix%20F%20%20Science%20and%20Engineering%20Practices%20in%20the%20NGSS%20-%20FINAL%20060513.pdf).
This activity includes practice in 1. planning and carrying out investigations, using mathematics and computational thinking, analyzing and interpreting data and engaging in argument from evidence. I use it in the beginning of the year in fourth grade to introduce these concepts and it's also a great activity to be left for a sub. EASY PREP! SUB READY!