What Latin phrase meaning 'something for something' describes a mutual exchange?

Which 19th century personality said, "You can fool all of the people some of the time, and some of the people all of the time, but you can't fool all the people all the time."?

President Harry Truman liked to say, "If you can't stand the heat, get out of ... what?

Who popularized the phrase 'survival of the fittest'?

Who is credited with the phrase 'Knowledge is power'?

What phrase meaning 'blank slate' comes from Latin?

Who said 'Ich bin ein Berliner' in a 1963 speech?