Oxygen..

[zebo-the-fat]

Also, if the air you breath has no oxygen content, your body would not alert you to the fact that you are suffocating. You just pass out peacefully and die.

This is true

I've always wondered why executions were not done with a CPAP mask and a bottle of helium that can be purchased at a party store.

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All birds, dinosaurs and their relatives do it better, even more distant relations of birds like alligators do it better. Most mammals do it poorly.

Another trivia, the gas exchange in a chicken happens inside its bones, abdominal cavity, as well as in its lungs. If you nick the main wing bone of a chicken, you can tape up the chicken's nose and beak, and it can still breath.

How many chickens did you have to nick to figure this out Chuck?

Helium is an expensive way to execute someone, why not use nitrogen, much cheaper. I saw a film where they had volunteers in a chamber with oxygen masks on, some had the oxygen reduced and replaced with nitrogen... they didn't notice, just went to sleep. A low cost and painless way to kill someone if that's what you really need to do.