you are a visitor at a political convention (or perhaps a faculty meeting) with n delegates; each delegate is a member of exactly one political party. it is impossible to tell which political party any delegate belongs to; in particular, you will be summarily ejected from the convention if you ask. however, you can determine whether any pair of delegates belong to the same party by introducing them to each other. members of the same political party always greet each other with smiles and friendly handshakes; members of different parties always greet each other with angry stares and insults.