A metaphor is created when a speaker says that S is P (word meaning) while communicating that S is R (speaker meaning).
In a way, it is
not saying (literally) what you mean. It always involves a discrepancy between
word meaning and
speaker meaning.
1. Sally is a block of ice.
Speaker meaning (Metaphoric meaning): Sally is emotionally unresponsive.
2. Richard is a gorilla.
Speaker meaning (Metaphoric meaning): Richard is big, fierce, nasty, and prone to violence.
3. You are a three-year-old.
Speaker meaning (Metaphoric meaning): You are mentally and emotionally immature.
[Extensive borrowing from the article
Metaphor, by John Searle, reprinted in the collection
Expression and Meaning]
Bottom line:
You are a three-year-old is a metaphor (provided it is not being applied literally to an actual three-year old).
CJ