My all-time fave serious short poem is a haiku by Basho:

An old pond;
Frog leaps in--
The sound of water.

(Furuike ya; kawazu tobikomu. Mizu no ono.)

Or for the comedically inclined:

Roses are red; violets are blue.
Some poems rhyme. Mine don't.

And yet another limerick not posted to the last limerick thread--for the landsmen among us a spanker is a kind of sail and the main sheet is part of a sailing ship's rigging:

There were two old ladies named Banker
Who slept while the ship was at anchor,
But they woke in dismay
When they heard the Mate say,
"Lift up the main sheet and spanker!"

CC