When you tell your students “You three work together”, there will probably be some among the majority who will complain and tell that they don’t want to work with A or B in that group. They will sulk, they won’t contribute, isolate themselves or even worse they won’t let the other members work. However; if you do it randomly, and if you use a different technique from time to time, the complaints will be less. I’d like to share some of the methods I use to group or pair my students.