The Butterfly Theorem = Circle + Chords + Midpoints
More details: The Butterfly Theorem: Proof

More puzzles:
  Puzzles: Table of Content
  Nazca Lines: Condor,
  Nazca Lines: The Monkey,
  Nazca Lines: The Spider,
  Machu Picchu,
  Sierpinski Triangle and Machu Picchu,
  The Stone of twelve angles,
  The Lord of Sipan,
  The Lord of Sipan: Spiderman,
  Butterfly Theorem,
  Bevan Point,
  Ceva's Theorem,
  The Raft of the Medusa,
  The Raft of the Medusa (Chinese version),
  The Raft of the Medusa(Spanish version),
  Euler's Formula,
  Morley's Theorem,
  Nagel Point,
  Newton's Theorem,
  Pentagons & Pentagrams