At (someone's) mercy phrase
Under the control of or controlled by the acts of another, without the power to protect or liberate oneself.
He pleaded guilty and he was at the court's mercy.
Peter was kidnapped and briefly detained by a terrorist group. He was entirely at their mercy.
A little girl was at a mad man's mercy. We have to hurry up.
To have someone in a difficult situation in which you have complete power over them.
White lie
Ken: Yesterday I lost the watch that my girlfriend presented to me. Should I tell her this thing?
Lily: Certainly Not! Your girlfriend will be angry with you. Children and fools tell the truth.