"What goes up a chimney down, but can't go down a chimney up?" Nearly everyone knows a few modern-style riddles—questions with humorous answers to be guessed by listeners, often involving a pun but sometimes consisting of sheer nonsensical silliness. Modern riddles are often collected in volumes with humorous illustrations, and evocative titles like 1001 Riddles. There are monster riddles ("What did the Frankenstein monster do when the MC said to give the little girl a great big hand?"), pickle riddles ("What's green and flies?"), meta-riddles ("What's the difference between a whale and a toaster?"), and plenty of other themes ("What's brown, lives in a tree, and is extremely dangerous?"). Riddles are often considered kid stuff these days, and are rarely asked with the serious expectation that a listener will figure out the answer; usually the intended answer is obscure, silly, or convoluted enough that only those already familiar with it will guess it.
But it hasn't always been thus. Readers of Tolkien will recall the chapter of The Hobbit entitled "Riddles in the Dark," in which Gollum and Bilbo try to guess each others' riddles in a life-and-death struggle of wits. This kind of riddle is not meant to be funny; it's a puzzle presented in metaphors. Tolkien, a philologist by trade, was hearkening back to Anglo-Saxon riddles, a verse form providing a set of clues to the speaker's meaning. (Though it's not clear whether riddle contests like the one Tolkien describes were actually part of Anglo-Saxon culture.) The clues often had an obvious off-color interpretation, and a less obvious non-ribald metaphorical interpretation to be guessed. The riddles often ended by telling the listener to guess the answer: "Say what I mean."
Anglo-Saxon (a.k.a. Old English) riddles, such as the hundred or so listed in the thousand-year-old book known as the Exeter book, were written in the standard form for Anglo-Saxon poetry: four or five stressed syllables per line, with any number of unstressed syllables between them (usually in no regular metrical pattern); at least two of the stressed syllables on each line alliterate with each other, usually with at least one syllable in the first half of the line alliterating with at least one in the second half. (If I recall correctly, all vowels counted as alliterating with each other.) For instance, one of the Exeter book riddles contains the line "flode ond foldan, / ferende gaest"—four stressed syllables, three of which begin with a /f/ sound.
Unfortunately I've neglected to obtain permission to print modern translations of old riddles, so you'll have to settle for a couple of modern-English riddles I wrote seven or eight years back. The first attempts to bring Anglo-Saxon verse patterns into modern English:
I heard of an invading, vanquishing army
sweeping across the land, liquid-quick;
conquering everything, quelling resistance.
With it came darkness, dimming the light.
Humans hid in their houses, while outside
spears pierced, shattering stone walls.
Uncountable soldiers smashed into the ground,
but each elicited life as he died;
when the army had vanished, advancing northward,
the land was green and growing, refreshed.
And the second ignores alliteration in favor of a double entendre:
I saw a strange creature:
Long, hard, and straight,
Thrusting into a round, dark opening,
Preparing to discharge its load of lives,
Puffing and squealing noises accompanied it,
Then a final screech as it slowed and stopped.
Say what I mean.