With regard to "table leg," we all have experience with legs, so in calling the thing that supports a table a "leg," we know immediately that it is long, thin, straight, and that it holds the table up, just as a leg holds up a body. "Old flame," on the other hand, reflects an emotional knowledge -- the way we experience love as a burning sensation, or a kind of heat. (And, in fact, many of our metaphors about love involve the idea of heat: someone's eyes might "smolder," Johnny Cash sang of a "ring of fire," etc.) In everyday speech, many metaphors are so subtle that we use them without realizing it.
Examples of common similes (in case you wondered) include "fit as a fiddle," "right as rain," and "sharp as a tack."
But do these kinds of everyday metaphors belong in your fiction? It's a question well worth asking, as a cliched or mixed metaphor can sink a perfectly good story. Avoid the two biggest traps with "Use Metaphors Correctly," and see how a bad metaphor can perfectly fit a comic situation with an example of a bad metaphor from George Saunders' "Pastoralia.")
