Riddle: what creature has four legs in the morning, two in the afternoon, and three at nighttime?
Answer: A Human. This is how it works:
In the moring of their life humans are babies and crawl on all fours, thus making them walk on four legs in the morning of their life.
In the afternoon, you stand up right on your two legs, making you have two legs in the afternoon of your life.
Nighttime represents someone's old age. They walk on two legs, but the third is their cane!