combinatorics - How does this image prove the identity $1+2+3+4cdots + (n-1) = binom{n}{2}$?

Proof without words:

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How does this image prove the identity $1+2+3+4\cdots + (n-1) = \binom{n}{2}$?

I found this here; could anybody explain this in a lucid manner?

Answer

This shows that every yellow circle uniquely determines a pair of blue circles and vice versa. The number of yellow ones is the LHS, the number of pairs of blue ones is the RHS. Cute!