Ingredients:
1 larger cooler (like rubbermaid or one of those)
1 small critter keeper
1 small block of dry ice (can bet bought at Publix or other supermarkets)

Take the small block of dry ice, and put it in the critter keeper. Put the critter keeper in the large cooler. Take your mouse/rat and put it in the cooler, and close the top. Wait 10-15 mins and your mouse/rat should go to sleep and die peacefully in it's sleep.