Cross (one's) heart (and hope to die) (Short video)

The phrase of the day is: "Cross (one's) heart (and hope to die)"

• You can say 'cross my heart' when you want someone to believe that you are telling the truth.
  [ --- I didn't take it, cross my heart! --- ]
  
• You can also ask 'cross your heart?', when you are asking someone if they are really telling the truth.
  [ --- I promise that I will pay you back the money next week. Cross my heart and hope to die. --- ]