On my way to work, I found an eastern garter snake in the street. It was still alive and barely moving. Its entire tail, up to about 1/3 of its body length, was flattened due to somebody running it over. How it got in the road in 39 degree weather is beyond me. Anyway, I put it in a cardboard box from work and it's now sitting in the office with me. There's a bit of blood and I'm pretty sure some organs were run over. Just when I think the snake has died, it moves in the box a little bit. I don't want this poor thing to suffer any more, and I don't have the money to get it to a vet. So, I think the best thing to do is to put the animal out of its misery. I know that beheading works, but I've seen snakes live for several minutes after losing their heads, so I don't think is a fair way.

Is there a quick, painless way of euthanizing a snake that obviously isn't going to live much longer and is in misery? Thanks.