“Does a one-legged duck swim in circles?”

Source: The Lightning Thief

1986 statement as quoted in "Long line of princely gaffes", BBC News (1 March 2002)
1980s

Regarding Communists; hearing before the Senate Committee on Labor and Public Welfare (1953)

Last Week Tonight (8 June 2014)
Last Week Tonight (2014–present)

Song lyrics, Aerial (2005), A Sky of Honey (Disc 2)
Context: The stars are caught in our hair
The stars are on our fingers
A veil of diamond dust
Just reach up and touch it
The sky's above our heads
The sea's around our legs
In milky, silky water
We swim further and further…

His collected works contain no riddle about dog legs, but George W. Julian recounts Lincoln using a similar story about a calf in Reminiscences of Abraham Lincoln by distinguished men of his time (1909), p. 241: "There are strong reasons for saying that he doubted his right to emancipate under the war power, and he doubtless meant what he said when he compared an Executive order to that effect to 'the Pope’s Bull against the comet.' In discussing the question, he used to liken the case to that of the boy who, when asked how many legs his calf would have if he called its tail a leg, replied, 'Five,' to which the prompt response was made that calling the tail a leg would not make it a leg."
A very similar riddle about cow legs was also circulated by Edward Josiah Stearns' Notes on Uncle Tom's Cabin (1853), p. 46: '"Father," said one of the rising generation to his paternal progenitor, "if I should call this cow's tail a leg, how many legs would she have?" "Why five, to be sure." "Why, no, father; would calling it a leg make it one?"'
Misattributed

Jace to Clary, pg. 83
Source: The Mortal Instruments, City of Bones (2007)

Source: Transmetropolitan, Vol. 9: The Cure