My rat has a large tumor in her armpit. The vet says as long as she's still eating, moving around, and doesn't act like she's in pain, just let it be. Thankfully she's still healthy, but I realize she probably won't be here at the end of the year. I've decided I don't want her to be put down with an injection because it's very painful for small animals. What are some other ways a vet might humanely euthanize a rodent?