As in the riddle of the Sphinx, the "two" here stands for youth
with its two sufficient legs, and the "three" for old age, which
requires a third support in a staff.
There were two things which God first thought of creating on the eve of
the Sabbath, which, however, were not created till after the Sabbath had
closed. The first was fire, which Adam by divine suggestion drew forth
by striking together two stones; and the second, was the mule, produced
by the crossing of two different animals.
THE TALMUD, _P'sachim_, fol. 54, col. 1.