“It almost made me long for the flying pig.

--Percy”

Source: The Last Olympian

Variant: Never seen Jason fly before," Percy grumbled. "He looks like a blond Superman
Source: The Mark of Athena

Initially attributed to Cyrus S. Ching in Time, Vol. 56 (1950), p. 21.
Misattributed
Variant: Never wrestle with pigs. You both get dirty and the pig likes it.

Variant: Why do you need to gallop while you fly?"
"Why do humans have to sway their arms while they walk? I dunno boss, but it just feels right.
Source: The Last Olympian

Review of 'What Darwin Got Wrong' by Jerry Fodor and Massimo Piattelli Palmarini (2010) http://www.theguardian.com/books/2010/feb/06/what-darwin-got-wrong.

On the USA, said during his exile in Peking, as quoted by Oriana Fallaci (June 1973), Intervista con la Storia (sixth edition, 2011). page 112.
Interviews

Source: The Blood of Olympus

Civil-suit deposition against the Herring-Curtiss Company (1909), reported in The Dayton News (31 May 1912) http://home.dayton.lib.oh.us/archives/wbcollection/wbscrapbooks1/WBScrapbooks10079.html
Context: My brother and I became seriously interested in the problem of human flight in 1899... We knew that men had by common consent adopted human flight as the standard of impossibility. When a man said, “It can’t be done; a man might as well try to fly,” he was understood as expressing the final limit of impossibility. Our own growing belief that man might nevertheless learn to fly was based on the idea that while thousands of the most dissimilar body structures, such as insects, fish, reptiles, birds and mammals, were flying every day at pleasure, it was reasonable to suppose that man might also fly... We accordingly decided to write to the Smithsonian Institution and inquire for the best books relating to the subject.... Contrary to our previous impression, we found that men of the very highest standing in the profession of science and invention had attempted to solve the problem... But one by one, they had been compelled to confess themselves beaten, and had discontinued their efforts. In studying their failures we found many points of interest to us.
At that time there was no flying art in the proper sense of the word, but only a flying problem. Thousands of men had thought about flying machines and a few had even built machines which they called flying machines, but these were guilty of almost everything except flying. Thousands of pages had been written on the so-called science of flying, but for the most part the ideas set forth, like the designs for machines, were mere speculations and probably ninety per cent was false. Consequently those who tried to study the science of aerodynamics knew not what to believe and what not to believe. Things which seemed reasonable were often found to be untrue, and things which seemed unreasonable were sometimes true. Under this condition of affairs students were accustomed to pay little attention to things that they had not personally tested.

Source: Catch a Mate