When I worked in a pet shop, we occasionally had to kill mice or rats that got ear infections and began circling or rolling non-stop, unable to eat or sleep. We put them in a plastic bag with very little air, closed it tight, and put them in the freezer. We told ourselves it was quick and relatively painless. It certainly worked.
I tried drowning one once, on the theory that it would be quick. It wasn't.
I tried drowning one once, on the theory that it would be quick. It wasn't.