One of the oldest known riddles is the riddle (ainigma) of the Sphinx. In ancient Greek tradition the Sphinx devoured all those who failed to solve it, but she destroyed herself instead when Oedipus got the right answer.
In its best known form, it runs as follows:
What walks on four legs in the morning, two legs at noon, and three legs in the evening?
The answer is man: a person crawls on all fours when a baby, walks upright in the middle period of their life, and uses a walking stick when they are elderly.
What alternative solutions can you come up with?
Having been advised that the question as phrased above is too broad, and after imagining theatre-goers storming out of a production of Sophocles's Oedipus the King, loudly objecting that "The Sphinx's puzzle was completely unrealistic. It's obvious there's an infinite class of possible solutions, consisting of any quadruped entity that loses two legs and then gains back one", I am adding the following restriction:
please ensure that at least one interpretation of the word 'leg' is non-literal.
In response to votes to keep this question closed for still being "too broad" even after the addition of the first restriction, the original poster has decided to add a further restriction as follows.
Since this question is not considered reopenable as it stands, for being too broad, please make all answers refer either to the behaviour of vicunas (aspects of their behaviour not also displayed by other camelids) or to the developing relation between advertising, participatory commercial ('social') websites, right-wing libertarianism, the work culture of computer programmers, schizophrenia, bureaucracy, and the function of ritual respect-paying to 'good faith', but not both.