This was written by my friend, cognitive scientist Daniel Saunders:
What walks on four legs in the morning, two legs at noon, and three legs in the evening?
Bearing in mind that when I refer to times of day, they could metaphorically stand in for the successive stages from the beginning to the end of a process, not necessarily occupying the span of one day, but potentially even years.
Also, when I say "legs", I may actually be referring to the broader class of things that place pressure against the ground to aid in walking.
Feel free to use broad generalizations about the species in your answer.