Unplugged (Case Study 2)

An Engaging Way to Introduce Computing

Computing is often a mystery: While people may know how to use computers, they rarely know what makes computers work. “CS Unplugged” uncovers the mystery by exposing students to computer science concepts, such as the nature of data or how data is sorted, but without the computer. The activities in “CS Unplugged” help to shatter the image of computing as long, lonely hours in front of an LCD screen by exposing learners to the kind of reasoning needed for inventing the next great ideas in computing.

“CS Unplugged” activities engage students in learning computer science concepts using hands-on activities. The activity described here, “Sorting Network,” illustrates the structures used in parallel sorting networks, exposing learners to sorting, parallelism, and binary comparison through active, kinetic learning. In teams of six, students compare numbers (small or large) and follow simple logic.


Start by drawing the layout to the right on the ground, using chalk on a pavement, masking tape for indoor surfaces, or electrician’s tape on a tarpaulin. Each student on the team holds a card with a number on it (for the first time, use the numbers from 1 to 6). The goal is to get the numbers sorted into order.

Each student stands on one of the squares on the “in” side of the diagram. Students follow the arrow to step onto the first circle, where they meet another student and compare numbers. The student with the smaller number follows the arrow out on their left, while the student with the larger number follows the arrow out on their right.

Students continue following the arrows to each circle as another student steps to the circle, each time comparing numbers. The smaller always goes left and the larger goes right. Eventually they will reach the “out” side in sorted order. (The full lesson plan, “Beat the Clock: Sorting Networks” can be found on the website described below.)

The exercise can be extended in a number of ways. For example, students could be timed to discover how quickly they can complete the sorting. For this, use larger numbers so it is hard to see where you are supposed to end up. And there are many questions to ponder: What if the smaller one goes to the right each time? How would you design a layout for sorting three numbers? Thirty numbers? Does it work backwards? Can you design a smaller layout to find the smallest number?


When three pairs of students are comparing numbers at the same time, it takes much less time than comparing only one pair of numbers at a time. This “Sorting Network” demonstrates parallel computation, one of many ways that computer scientists have devised to sort data quickly. Instructors tell students that they have just learned about the computing concepts behind computer applications with which they are familiar, such as alphabetical lists of files, etc.

Initial evaluations of sessions involving this activity and others show that children gain a better appreciation of what Computer Science is about, and girls in particular respond positively to the logic and problem solving. More detailed international evaluations are underway.


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Authors: Lecia Barker and J. McGrath Cohoon