When will they collide?

Show the video to your class. Give students time to talk about what information they'd need first.  Then take guesses from students about the actual time it would take to collide.

 After students have worked for a while and you have traveled around and selected some students and their work to share, reveal our answer to this question.

Thanks to our friends Kim Lequire and Kyra Mercado for telling us about this video.