Code Making and Breaking are a great hook for talking about functions, inverse functions, one-to-one functions etc. I've gotten great mileage out of this worksheet in every class I've used it, from honors to general track. Next week I am going to try it out with my students here in China. I encoded the captions to the four Far Side cartoons using a simple substitution cipher –like you would find in a newspaper cryptogram. I think the cartoons make the exercise more engaging.
Below is the slide I use to introduce the exercise. The carrot cartoon I chose is pretty dark but Larson's Far Side comic is often dark. (I don't think any of the captions or cartoons I chose for the worksheet are as dark.) Some students have actually never seen the Far Side before! Students usually initially think this task is impossible and usually let me know this. I am relentless, I encourage them to look for a "door-way" that will let them into the puzzle. The two most obvious entry points are in comics one and four. They feel really accomplished when they crack the codes. A nice extension activity for keen students is to have them find their own comic code it similarly.