dehydrated baby mouse

Question
Hi, I bought 2 albino mice from a pet store. One is young but fine and the other is tiny and not acting right. It is a lot smaller so it must be a baby but when I brought them home they both had wide bright eyes and everything was fine. 2 days later the little one was in the food bowl almost lifeless. I touched it and it didnt move and after trying to pick her up (thinking she was dead) she moved. However her eyes never opened. I have always had hamsters and I do know that dehydrated hamsters will walk around in a daze with their eyes closed (found this out by mistake). I am wondering if mice will do this too??? I have been putting the water bottle up to her mouth and she will drink it (every 3-4 hours) but her eyes are still half closed. She is eating fine tho. She isnt very active but still eating and will drink if I squeeze the water bottle for her to lick. I am wondering if she is too young for her be able to get the water bottle ball to move for her to get water???  Wut to do??? Thanks for your advice

Answer
She is either the runt of the litter or too young and should not have been sold to you. It is common for baby rodents to go downhill from the shock of moving and rarely survive away from the litter.
Your mice should be at lease 6 weeks old to be fit for sale as at this age they are able to breed and are weaned.
There is a chance that the little one is not fully weaned and is only 3.5 weeks old in this case it should be with its mum and drinking milk.

There is little chance of her surviving if she is this age but you could go to your vet and get some critical care formula which has ingredients to boots her immune system and vitamins.

Another thing she could have if she is losing weight and her vent is wet is wet tail like hamsters get or tyzers disease both ailments will need vet treatment