And none will hear the postman's knock Without a quickening of the heart. For who can bear to feel himself forgotten?

-Auden,W(ystan) H(ugh)
  'Night Mail', written to accompany a documentary by the Post Office Film Unit.

   Ask, and it shall be given you; seek, and ye shall find; knock, and it shall be opened unto you: For every one 110 that askethreceiveth; and hethat seekethfindeth; and to him that knocketh it shall be opened.

-Bible (NewTestament)
St Matthew 7:7^8.

Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me.

-Bible (NewTestament)
Revelation 3:20.

Batter my heart, three-personed God; for You As yet but knock, breathe, shine, and seek to mend.

-Donne,John
c.1610^1615  Holy Sonnets, no.14.

You have a row of dominoes set up.You knock over the first one, and what will happen to the last isthat it will go over very quickly.

-Eisenhower, Dwight D(avid)
  Explaining the domino theory in relation to SE  Asia,7  Apr.

Every man has a right to utter what he thinks truth, and every other man has a right to knock him down for it. Martyrdom is the test.

-Johnson, Samuel known as Dr Johnson
  Quoted in  James Boswell The Life of Samuel Johnson (1791), vol.4.