"What goes on four feet in the morning, two feet at noon, and three feet in the evening?" (H&P give essentially the same version, p. 637, but with "legs" in place of "feet".)
"A thing there is whose voice is one;
Whose feet are four and two and three.
So mutable a thing is none
That moves in earth or sky or sea.
When on most feet this thing doth go,
Its strength is weakest and its pace most slow."
Besides this relatively comprehensive version, there is also evidence for a shorter version, consisting of just one dactylic hexameter line, as follows:
"A thing there is whose voice is one;
Whose feet are four and two."
One possible answer to this form of the riddle is "a pastoral society" (i.e., one in which humans and their animals live in close association with one another). Such a formulation of the riddle is important at various points in Oedipus Rex. For example, the plague is described near the beginning of the play (H&P, p. 650, lines 24-25) as affecting both the flocks and women of Thebes. Also, it is eventually the two shepherds (from Corinth and Thebes respectively), who have lived in close association with their flocks (H&P, p. 685, lines 1082-1090) who eventually provide the key evidence for explicating Oedipus' background.
More generally, evidence for the importance of a variety of different forms of the riddle emerges in the confrontation between Oedipus and Teiresias (H&P, pp. 657-663, lines 289-453). This can be viewed in terms of Teiresias' having seen that the riddle did not admit of any simple solution, whereas Oedipus, brilliantly, but with ultimately disastrous consequences, picked out just the answer "man".
Particularly striking evidence of the importance of a multiplicity of riddles in the play comes in the concluding lines, in which, finally, there is a reference to riddles in the plural rather than a single riddle. This point is, however, obscured in many translations (including Cook's translation, in H&P, p. 697), in which the Greek plural ainigmat(a), which Sophocles uses in place of the singular ainigma, is translated just as "riddle". For the original form of the text, though, see the following sources (emphasis added in each instance):
Translation by David Grene, available near the end of an on-line essay:
Overall, it is probably the case that the translation of Sophocles' word ainigmat(a) as an actual plural "riddles" is more often found in treatments of Freud and his theory of the "Oedipus complex" than in more strictly "literary" approaches.
One such site provides the translation by Jean Grosjean (about a third of the way down the webpage):
In the Freud bookplate, the Greek word ΑΙΝΙΓΜΑΤ constitutes the second line of the Greek text.