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)