Grade 9: Neutrinos
Neutrinos follow a straight line through a 10×10 grid of atoms each with radius 0.1. Which atoms do they hit, if any? We then work with a 100×100 grid of atoms each with radius 0.01. The students will work with a spreadsheet and will need a precise geometric condition to decide whether a given atom is hit. That involves them in a Pythagoras and similar triangle argument, so this could also work well in grade 10.