This is a lesson I use to introduce the definition of function. It is one of my students favorite lessons, and gets the students out of their seats and involved. It also comes with what I call "the easiest math worksheet you will ever do." If you want to get this activity for free check out my website: https://domathbetter.weebly.com/fun-stuff.html
Students answer a variety of 6th grade level math problems to find the secret message. Once they have found the message the remaining numbers will give the first 18 digits of Pi.