The theorem of friends and strangers can eb explained by using the Facebook analogy. Given six people on Facebbok and you’re one of them, you can either be connected or not with any of the other five. Between these six persons you can draw lines (red – not friend, green -friend) and you get 2^15 = 32 768 different combinations of all these 15 lines.
What can we say about these combinations? Can we get some rules out of this chaos? It turns out that given these six persons there will be always 3 people that are friends OR always 3 people that are not friends. Simon Pampena explains why in the video from above.