Place the neonatal animals in a plastic bag and without squeezing them, squeeze all of the air out of the bag. Refill the bag slowly with CO2, doing it slowly ensures you do not freeze the animals. Repeat this step two more times to ensure the environment is 100% CO2. Once the bag is full of CO2, seal it and wait until the pups have been motionless and lose color for at least a half an hour. ( Sundberg, J. & Boggess, D, p. 68-69 ) It may be necessary to clip the spine with sharp scissors immediately after removing the animals from the bag to ensure death.