Mix them up randomly. Warn them NOT to open the lid. Their task will be to shake the container and listen to the sound it makes. When the match is made, the pair should sit down next to each other. The teacher should time the activity to see how efficient the class members are at finding their match.
Many animals rely on sounds to communicate to others in their species.
Technique of shaking will vary and you may wish to point out how holding the container by placing a finger on the cover may dampen or change the sound. I've observed that many students take both containers and shake them themselves in an unconscious effort to standardize the procedure. I actually use this activity to discuss experimental procedure by pointing out how scientists are seeking the unknown and have to eliminate data that obviously does not meet criteria and then work to refine the remaining possibilities. We also talk about safety in working with unknowns and how we must use our senses and tools available to us. Soak the cotton balls in the scented extracts.