The Challenge: Drawing only straight lines where all lines must cross all other lines, how many can you draw?

Materials needed: Paper, pencil, maybe a ruler.

(As I allude to with my warning -mostly for twitter folk- there is an optimal solution for this, and it’s very satisfying if you can find it on your own. If you want it spoiled for you, look below.)

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The optimal solution to this ends up forming a parabola.

So middle school teachers, this is a great way to introduce slope, because a successful image has all different slopes.

Early high school teachers, you can use this to encourage students to practice generalization skills in Desmos.

Calc teachers: here’s your intro to derivatives as slope. The derivatives are drawing the function for you.

And if you’re nervous about materials, feel free to ask students to fold this from any old piece of paper.