Comparing finished classifications is part of the lesson and students are curious about the results of different groups. Some duplication within each set is useful, too, because grouping buttons that are exactly alike is an easy first step toward engaging students in the activity. Introducing the Exercise Mystery heightens interest, therefore I keep the introduction brief. I include the color categories to give students a hint for their own work. However complex the example is, it is important to stress that the top label must include all of the items the students are working with.
Consistency is also important.
Each group receives an envelope with their set of items and a piece of A3 paper so they have lots of space to organize and label their classification. Assorted Applications Vocabulary is a good focus to use with low intermediate students, and this activity can be linked to a textbook unit on descriptions. It's a shank . . . Grouping words into categories is itself a mnemonic device, and leads students to more and more abstract terminology. Advanced university English students did this exercise as part of a unit on critical thinking.