They put them in a sealed chamber and then vacuum the air out quickly so they suck the lungs flat.
I forgot to add - after they kill the dogs the put them in a big oven sort of thing and burn them up - you can smell burning hair and meat - they cremate them - no room the bury them - there are just too many - every kill day the women who worked there were in tears all day.
There are a great many ways to rescue dogs. If we stop buying they will stop breeding - put the puppy mills out of business. I do not care what the reasons anyone might have for buying a dog. There are millions of free dogs waiting for a home.
Sorry I am being mean and rude but we worked on building that place for about 6 months and I saw a lot of kill days. I know people who work in NO KILL places but it is still very hard for them to find people who want to adopt. And then we see so many breeders who just breed to make money while so many are already out there waiting to be adopted. Buying encourages breeding. It needs to stop.