Among Six People, Either Three Know Each Other or Three Are Strangers to Each Other
Represent the six people as the vertices of a regular hexagon. Draw a blue segment between a pair of vertices if the corresponding pair of people know each other and otherwise draw a red segment. A triangle with all three sides of the same color is called monochromatic; fill in such triangles with the appropriate color. There is always a monochromatic triangle! Moreover, there are always two monochromatic triangles!