These analogies come in the form of similes -- an expressly made comparison using "like" or "as." A metaphor condenses the comparison and omits those explicit signals of comparison.
 
Rev. Martin Luther King, Jr., wrote his famous "Letter from a Birmingham Jail" in response to a letter from 8 clergymen, published in the Birmingham, Alabama newspaper , that had urged Rev. King and his movement to cease their peaceful protest of racial segregation in public facilities and obey police and the courts. Rev. King was in jail at the time because he was held in contempt for having violated a restraining order against a protest march, issued under a local court's interpretation of the local parade ordinance. While Rev. King's incarceration for contempt was upheld by the Supreme Court, Walker v. Birmingham, 388 U.S. 307 (1967), the Court struck down the denial of a parade permit to allow the protest a violation of the First Amendment. Shuttlesworth v. Birmingham, 394 U.S. 147 (1969). Rev. King's letter has been reprinted widely, and can be found in Martin Luther King, Jr. Why We Can't Wait 76-95 (1964); Martin Luther King, Jr., I Have a Dream: writings and Speeches that Changed the World 85 (James M. Washington ed. 1992); A Testament of Hope: The Essential Writings of Martin Luther King, Jr. (J.M. Washington, ed. 1989); and 26 U.C. Davis L. Rev. 835 (1993). For a discussion of the circumstances under which King wrote his letter, see Taylor Branch, Parting the Waters: America In the King Years 737-44 (1988). For a general discussion of the civil rights struggle, see Juan Williams, Eyes on the Prize: America's Civil Rights Years (1987).