Video from Ted-Ed
"Why can we find geometric shapes in the night sky? How can we know that at least two people in London have exactly the same number of hairs on their head? And why can patterns be found in just about any text — even Vanilla Ice lyrics? PatrickJMT describes the Ramsey theory, which states that given enough elements in a set or structure, some interesting pattern among them is guaranteed to emerge. Lesson by PatrickJMT, animation by Aaron, Sean & Mathias Studios." from video introduction.