Lesson
"Cross one's heart and hope to die" - {v. phr.}, {informal} To promise somebody very seriously that everything you said is true.
Children often use it to ensure somebody that they are not lying . Children often make a sign of a cross over the heart as they say it, for emphasis. "Cross my heart, I didn't take your ball," Jack told Sam. "I didn't tell the teacher what you said. Cross my heart and hope to die," Jane said to Sandy.
