The Bible is Full of Riddles

I am studying through the Proverbs on Monday mornings with some of our neighbors in need in Simi Valley before distributing food, clothes and other necessities to them. Reading through the Proverbs is a great way to impart wisdom to people who have experienced poverty from injustices, laziness, lack of understanding, interpersonal conflicts, addictions, and a whole host of other causes. The Proverbs have so much to say about human relationships and, ultimately, our relationship to God and how that affects all other relationships. They also give us great insights through observations of everyday occurrences which are just as relevant today as they were thousands of years ago. 

We are nearing the end of the Proverbs after almost a year and have come upon some very interesting riddles. In the final few chapters we transition away from Solomon and into hearing from a man named Agur son of Jakeh. I want to post one here to see if you can discern it's meaning.

Three things are too wonderful for me;
four I do not understand:
the way of an eagle in the sky,
the way of a serpent on a rock,
the way of a ship on the high seas,
and the way of a man with a virgin.
(Proverbs 30:18-19 ESV)

So, what do you think? What is the hidden meaning? What do all these have in common?