The Surprise Symphony reference article from the English Wikipedia on 24-Apr-2004
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Surprise Symphony

The Surprise Symphony is another name for Joseph Haydn's symphony no. 94. Haydn was well known for putting "jokes" in his music, and this symphony is no exception: He inserted a loud "surprise" chord near the beginning of the second movement after a relatively tranquil opening. After the chord, the music immediately quiets down again, leaving the audience bewildered.