Pythagorean Theorem
"Proofs of Pythagorean Theorem" - by Nikola Petrov, University of Oklahoma - http://www2.math.ou.edu/~npetrov/pythagorean_theorem.pdf
Euclid's Elements, Book I, Proposition 47 - http://aleph0.clarku.edu/~djoyce/java/elements/bookI/propI47.html
"Neptune - In Depth" - NASA Solar System Exploration - http://solarsystem.nasa.gov/planets/neptune/indepth
"Ten of the greatest: Maths puzzles" - Daily Mail - http://www.dailymail.co.uk/home/moslive/article-1284909/Ten-greatest-Maths-puzzles.html
"How does GPS work?" - physics.org - http://www.physics.org/article-questions.asp?id=55
"Jaquet-Droz Automata" - Atlas Obscura - http://www.atlasobscura.com/places/jaquet-droz-automata