How to tell if I bought my mouse to young?

Question
My boyfriend and I bought two mice at the pet store the other day, his is bigger than mine and seems much more energenic. All it seems to be doing is makeing a clicking sound. I read on another page that that sound means it is hungry. Although it seems to be drinking water and eating food all yesterday and it has pooped and urinated but how often I am not sure. Last night I picked up her house and she was just their didn't seem like she was breathing very well. We figured she needs the mothers milk and that it was going to pass in the night. The other mouse seemed to be covering it up or somthing. Pushing shavings onto it and cleaning it and it seems to be comforting it.
This morning when I woke up I thought I would it would be dead but it wasn't, I lifted the house it got up cleaned itself ran around a little, didn't eat or drink, and went back into the house.
I have never had mice before and I am not sure if it is too young or if it is okay.
It's eyes don't seem to be widely open, they are not popped out or looking really healthy.
I can't go to a store to get this KMR milk and all I have is canned cows milk. Is there anything I can do, or should I take it back to the pet store before it gets worse?
Do you think it might survive?

Answer
Dear Monika,

The smaller mouse is either sick or too young.  The partially opened eyes are a sign of either one.  I do recommend you take it back to the pet store and get a healthy one. You can try to feed it as well as you can, but the problem might not even be age.   

I don't know of a clicking sound that means hunger.  If there is food in the cage, your boyfriend's mouse should not be hungry.  There must always be plenty of food in a mouse cage.  Mice almost never overeat.  

Please write again with any other questions.

squeaks,

Natasha