This works wonderfully. 1. Small container, holes melted with solder gun in lid and right below rim of container: 2. Larger container, holes around the top of the container for expanding gas to push out O2 (lid not pictured): 3. Place mice in larger container with lid on. Place Dry Ice in smaller container and put lid on. Run warm water into dry ice container - it will fall through the holes in the lid and begin to spew forth lots of CO2. Quickly place dry ice container into larger container and close lid. It doesn't take much dry ice. About the size of a piece of bread will give more than enough. My Kroger sells it for $1.68 a pound. So a piece the size I need is less than a dollar. To me, this is much easier than working with CO2 cannisters and regulators and hoses. The only drawback is a trip to Kroger right before each euthanization. However, Kroger is only 10 minutes away, so it isn't a problem for me. And I was worried about the dry ice "melting" too quickly. A piece the size of your palm will last quite a while in the open air. An hour or two.