At what age do you explain death to your children?

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My three year old daughter has started saying her brother is dead when he is asleep. She also says it when someone on tv is sleeping. I am not sure where she picked this up we sensor everything she watches but some things slip through. She even pretends to cry when she says it. I don't know what I am suppose to say to her. Am I suppose to explain what daeth is to her at three years old? I don't know if she would understand even if that was the right thing to do. It just breaks my heart to know she has learn about it some time and I really don't want to have to tell her. CAn anyone give me some advice about what I need to do?