For those who don't know, or may have forgotten, a chiasmus is a grammatical figure by which the order of words in one of two parallel clauses is inverted in the other. That's how the OED defines it, and that's pretty much the clearest definition I could offer up.
More directly, JFK's famous, "Ask not what your country can do for you; ask what you can do for your country," is a great example of a simple chiasmus. (You pronounce it kahy-az-muhs, by the way, and it comes from a Greek word meaning "to invert.")
Winston Churchill had a few good ones, too:
"Mankind must put an end to war,
or war will put an end to mankind."
"We shape our dwellings,
and afterwards our dwellings shape us."
"The whole history of the world is summed up in the fact that,
when nations are strong, they are not always just,
and when they wish to be just, they are no longer strong."
Here's a good one from old Bill Shakespeare: "I wasted time,
and now doth time waste me."
And one from none other than Ben Franklin: "The heart of the fool
is in his mouth, but the mouth of the wise man is in his heart."
As always with the Word Wrap, we open it up to you. Can you come up with a good one of your own?