This short activity is a very fun way to introduce the importance of following directions in the classroom. I use it in my science classroom, but you can easily alter it to fit the subject you teach.
In short, some students will actually follow the directions. The other students will not follow the directions and painstakingly, and uselessly, try to completely each task before time is up. Meanwhile, you and the first students will silently snicker at one another.
It's a blast!