In Chinese Buddhist monasteries at the end of the final ceremonies of the evening, this verse is chanted:

This day is already done.
Our lives are that much less.
We’re like fish in a shrinking pond;
What joy is there in this?
We should be diligent and vigorous,
As if our own heads were at stake.
Only be mindful of impermanence,
And be careful not to be lax.
(Universal Worthy Bodhisattva’s Verse of Exhortation, TT 111)

The Buddha taught about impermanence in order to help living beings sever their attachment to the ideas of permanence and the eternal, particularly those taught by non-Buddhist religions.

The Buddha said to the king [Prasenajit], “May I ask, is your body as indestructible as vajra, or is it subject to decay?”

“World Honored One,this body of mine will continue to change until in the end it will perish.”

The Buddha said, “Your majesty, you have not perished yet. How do you know that you will?”

“World Honored One, my impermanent body, though subject to decay, has not perished yet, but now, upon reflection, I can see that my every thought fades away, to be followed by a new thought which also does not last, like fire turning to ash, constantly dying away, forever perishing. This convinces me that my body must also extinguished.”

The Buddha said, “So it is. Your Majesty, you are in your declining years. How do you look now, compared to when you were a boy?”

“World Honored One, when I was a child, my skin was fresh and smooth. When I reached the prime of life,I was full of life and energy. But now in my later years,as old age presses upon me,my body has become withered and weary. My vital spirits are dull,my hair is white,my skin is wrinkled, and I haven’t much time remaining. How can this be compared to the prime of life?”

The Buddha said, “Your Majesty, your body’s decline cannot have occurred suddenly.”

The king said, “World Honored One, the change has been so subtle that I have hardly been aware of it, since I reached this point only gradually through the passage of the years. Thus when I was in my twenties, though I was still young, I already looked older than I did when I was ten. My thirties marked a further decline from my twenties, and now, at two years past sixty, I look back on my fifties as a time of strength and health.

“World Honored One,as I observe these subtle transformations, I realize that the changes wrought by this descent toward death are evident not only from decade to decade ;they can also be discerned in smaller increments. If I consider them more closely, I see that the changes occur by the year as well as by the decade. In fact how could they occur merely year by year? There are changes every month. And how could they occur only by the month? These changes happen day after day. If one contemplates this in depth, one can see that there is ceaseless change from instant to instant, from thought to thought. Thus I can know that my body will continue to change until it has perished.… ”

The Buddha said, “…What changes will perish. What does not change does not come into being and does not perish. How could it be subject to your birth and death? You no longer need be concerned…that after the death of this body, there is annihilation.”

The king believed the words he had heard, and he knew now that when we leave this body, we go on to another.… (SS II 25-35, BTTS rev. tr.)

無明 [wu ming] ; avidyā ; avijjā