Similarity—the idea that two non-identical things are nonetheless related in some way—is a powerful concept. The idea seems natural to us because humans are pretty good at discovering similarities, drawing analogies, and creating metaphors. When we're told that two things are similar, we can usually draw some connection between them.
Given an analogy or metaphor, for instance, we quickly find reasons for comparison between its elements. If someone refers to "biting cold," we understand the implicit metaphorical idea that cold can feel like teeth biting into skin. And if someone remarks that March goes out like a lamb, we know they mean it's gentle. (Though as I pointed out to various friends during a snowfall about three weeks ago, this year March went out like a lamb in a different sense—it was fluffy and white.)
Lewis Carroll had no answer in mind when he wrote his famous conundrum "Why is a raven like a writing-desk?", but when readers started asking what the answer was, he came up with one: "Because it can produce a few notes, tho they are very flat; and it is nevar [sic] put with the wrong end in front." Others have come up with other answers over the years: "Poe wrote on both," for instance, and Sam Lloyd's marvelous "The notes for which they are noted are not noted for being musical notes."
A year or two back I came up with a parlor game in which one player states an obscure simile (such as "grain is like a radiator") and another must come up with a reason why the two items are similar. (Turns out Monty Python once did a sketch around a similar concept.) Diana Stiefbold independently developed a much better version of this game, known either as Analogies or Similes: everyone in the room writes a concrete term on one slip of paper and an abstract concept on another slip of paper. All the concrete terms are mixed together in one hat; all the abstractions are mixed in another hat. Someone draws one slip from each hat and reads the two terms aloud—"shoelaces are like tyranny because..."—and then everyone in the room tries to come up with reasons that the two things are similar. (The best answer to that particular analogy was from Fred Bush: "Both keep tongues in line.") When nobody can think of any more similarities between the terms, someone pulls two more slips from the hats. The game continues until players get tired of it or the hats are empty. It's a remarkable creativity-stimulator, though players sometimes don't come up with the best answers until much later. (The first time I played this game, we were asked to explain why snow is like sex. It wasn't until about two days later that what should've been the obvious answer occurred to me: it's often best to wear rubbers for both.)
Mark-Jason Dominus and Ranjit Bhatnagar created another game that explores similarities: Plenty Questions. Much as in Twenty Questions, one player thinks of an item, object, or concept, and other players attempt to guess what the chooser is thinking of. In this game, however, the guesses must be other objects or concepts (instead of questions about the nature of the item to be guessed), and the answers to the guesses are limited to "warmer" or "colder," which translate to "the guess you've just made is more/less like what I'm thinking of than your previous best guess." (The answer to the first guess is almost always simply "no.") Of course, similarity is in the mind of the beholder, and there are a lot of different ways in which concepts can be similar, so sometimes it can be hard to decide whether a new guess is more or less similar to the chosen object than the previous best guess; but that's part of the fun of the game, trying to figure out in what ways things are or aren't similar, and to what degree. For instance:
[Jenny has picked "bicycle" as the item to be guessed.]
Jenny: Hmm. I'd have to say a dolphin is warmer than a marshmallow. [Perhaps she's noting that a dolphin has moving parts, could theoretically be ridden, and is closer in size to a bicycle than a marshmallow is.]
Konrad: Epic poetry.
Jenny: Epic poetry is colder than a dolphin, so dolphin is still the closest guess so far.
Jenny: Warmer than a dolphin. [A human technological artifact—arguably true of a marshmallow or epic poetry as well, but not the way Jenny's thinking about it.]
Konrad: 1993 Subaru Impreza Wagon.
Jenny: Warmer than a refrigerator.
Louise: Monster truck rally.
Louise: Bicycle.Jenny: Yep, you got it.
Of course, you don't have to choose concrete objects. In the first game of Plenty Questions I ever played, Arthur Evans chose "patripassionism," an obscure Christian heresy. Guessing by myself I didn't get anywhere near the answer, but when a couple of other people joined in the guessing we got fairly close before our knowledge of obscure heresies ran out and we had to be told the specific answer.
(Thanks to Jim Moskowitz for his aid in ironing out this column.)