I vote in favor of telling her the truth and helping her understand it.Grief is the first step to recovery and it is natural. Of course, that doesnt mean, the process should not be eased by those who can help.Children, as paneer said, do not fully understand death as adults do, but the are more sensitive when we try to avoid an issue and "learn" that behavior. Death is part of life and in fact the reason we should live it fully and joyfully.My advice is to tell her that the cat will not come back, explaining it to her that it has left and will not return. (I am sure she has seen animals die too). Tell her, that the best way to keep her alive is by cherishing her feelings and sharing them with others (starting with you), because, above all, children are capable of having beautiful feelings. Maybe you will learn from her too...Just my 2 cents.