Then put markers of a second color on all the corners that are one street away from that ice cream stand. Remind students that part of the goal is to have as few stands as possible, because stands are expensive to build. After the students have had a chance to work on the puzzle and solve it, show them how it was made. Explain how you can make a puzzle like the ice Cream Puzzle by drawing the solution first. This looks easy.
Closure Have the students make a simple puzzle of their own, using the solution first method.
Explain to students that this puzzle is an example of what mathematicians call a one - way function. If you start with the solution and create the puzzle, it's easy. If you start with the puzzle and have to find the solution, it's not. Ask the students to think of other situations in real life that may present themselves as a puzzle like this. Some ideas could include hospitals, hamburger restaurants, firestations, restrooms, etc.