The comparison of one familiar thing with another in order to promote understanding has been a rhetorical device for thousands of years. The following are some of the most common comparisons.

Analogies

Analogies make comparisons between two sets of items: “shoe is to foot as tire is to wheel.” The purpose is to clarify the relationships; they do not have to be precisely the same, point by point.

An analogy could be made between two things, one of which is familiar, shedding light on the other: “a heart is like a pump” assumes that everyone knows how a pump works. The pairings can be antonyms, synonyms, descriptive (blue is to sky as red is to fire truck), part to whole (arm-body), or item to category (milk-beverage).

Metaphors

A metaphor implies that two very different things have the same properties and is more assertive than an analogy. It substitutes one for the other, making them essentially identical: “you are my sunshine.” A well-known extended metaphor is from Shakespeare's As You Like It:

All the world's a stage,
And all the men and women merely players; They have their exits and their entrances;
And as one man plays many parts,
His acts being seven ages. At first the infant,
Mewling and puking in the nurse's arms.
And then the whining school-boy, with his satchel,
And shining morning face, creeping like snail
Unwillingly to school. And then the love …

There are many types of metaphors. The answer to the famous riddle of the Sphinx is an example of a conceptual one, a metaphor that is systematic: What goes on four legs in the morning, two legs at midday, and three in the evening? A man — crawling as a baby, then upright, then using a cane in old age.

Similes

Similes are similar to metaphors, except that in a simile the comparison is made explicit by using the words “like” or “as.” Instead of “you are my sunshine,” a simile would state this as “you are like sunshine to me.”

Other common similes:

• Sly as a fox

• Like playing with fire

• Clean as a whistle

• Hard as nails

• Pretty as a picture

Hyperbole takes the comparison and exaggerates it. “I had to wait for what felt like forever” would be a simile, while hyperbole would phrase it as “I had to wait forever.”