One thing I notice is that the rat cooks slower, so if I put them in together, the mouse overcooks and probably busts a vein somewhere, because there's usually blood all over the nose and mouth. I think for the next feed I'll put the mouse and the rat together again, but the one after that I'll only give her the rat. I've tried just thawing the mice and heating them later, but for some reason the stomachs bloat and are prone to bursting, which is a hassle to clean up. Since then, I just put the frozen mouse in hot water until it's soft and warm. She usually takes it as soon as it's presented to her.