Okay I just tried this method on 4 mice and I have a question. The cooler I used is fairly large. I put some dry ice in a tupperware with a bunch of holes drilled in the lid. I opened the lid of the cooler after about 10 minutes and several of the mice appeared to be dead, but I noticed one of them lying there still breathing a bit. Does this mean that I didn't use enough dry ice, or was it too much? I think I had about 1 lb in there. No water. Thanks!