Opposite angles are equal
Take two intersecting lines:
\
A \ B
-------
C \D
\
A + B = 180 (def of line)
C + D = 180 (def of line)
B + D = 180 (def of line)
C + A = 180 (def of line)
Therefore
A + B = B + D (equality)
A = D (subtract B from each side)
Therefore oppsite angle are equal
Is that not a work of art? Its simplicity and elegance, one of the basic corner stones of Euclidean geometry.
(I make no claims as to its originality)