system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:When you ignite your propane barbecue, why does the flame not travel down the hose and into the tank?

I've never been able to figure this one out. There doesnt seem to be any little valve, just the hose attached onto the burner mechanism. Same with the patio heater. Yet the flame sits where it should and doesnt ignight the rest of the propane in the tank. Why? Is this simple physics or is there a mechanical device in there that I just don't see?

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Edit: thank you good people of Reddit, that makes perfect sense to me now. I wish I had asked ages ago.
gpt
The propane needs oxygen to be able to burn (burning is just chemically combining with an oxidizer), and there's no oxygen in the hose.  As long as the pressure is such that the propane is constantly pushed out, then oxygen can't get in, and the propane in the hose can't burn.