The following is from a Dharma talk on Buddha's concept of Impermanence by Fred Nelson, a teacher of the Des Moines Meditation Group. He introduced it by reading verses from the poem "On the Nature of Things" by Lucretius, a Roman poet who lived from 99 BC to 55 BC, and followed it with personal remarks on Impermanence.

 "On the Nature of Things" by Lucretius

No single thing abides; all things flow.
Fragment to fragment clings -- things thus grow
until we know and name them. By degree they melt,
and are no more the things we  know.

Globed from the atoms falling slow and shift,
I see the suns. I see the systems lift their
forms; and even the systems and the suns
shall go back slowly to the eternal drift.

Thou too, oh Earth -- thine empires, lands, and seas --
least with thy stars of all the galaxies,
globed with the drift like these, like these thou too
shalt go. Thou art going, hour by hour like these.

Nothing abides, thy seas in delicate haze go off;
those mooned sands forsake their place;
and where they are, shall other seas in turn
mow with their scythes of whiteness other bays….

The seeds that once were we take flight and fly,
winnowed to earth, or whirled along the sky,
not lost but disunited. Life lives on.
It is the lives, the lives, the lives, that die.

They go beyond recapture and recall,
lost in the all-indissoluable All:
gone like the rainbow from the fountain's foam,
gone like the spindrift shuddering down the squall.

Flakes of the water, on the water cease,
soul of the body, melt and sleep like these,
atoms to atoms -- weariness to rest --
ashes to ashes -- hopes and fears to peace.

Impermanence is evident if we look around. Rationally, scientifically, by observation, impermanence is a very simple concept. Accepting impermanence and its implications is another thing. The book that challenged me to accept impermanence was the book that introduced me to the Buddha's teachings: "Buddhism Without Beliefs" by Stephen Batchelor. The Buddha's awakening and acceptance of his life & death was very much about understanding and awakening to impermanence. 

This started with his seeing human impermanence in disease, aging, and death. The book is a clear introduction to the teachings of the Buddha.  The chapter on "Anguish" is introduced with: "No conditions are permanent; No conditions are reliable; Nothing (or no thing) is self," statements credited to the Buddha. This chapter and the chapter on "Death" are good readings that challenge us to first recognize our own impermanence. The meditation Batchelor introduces on human impermanence has been helpful: "Since death alone is certain and the time of death uncertain, what should I do?" 

Fortunately, the Buddha's teachings on impermanence are not a negative. They are an important realization and positive remedy for reducing human suffering.