system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some planes leave long white streaks in the sky and others don't? And what exactly is that gas?

edit: So, if I've learned anything from this, its that the clouds are chemicals the government uses to control us all. And anyone posting any other explanation is likely a government shill. Thanks Reddit!
gpt
That "gas" isn't actually a gas. It's really just ice that formed because of the combustion in the engines. The low pressure and cold temperature cause the water vapour to spontaneously depose into ice. This effect doesn't happen when for example the pressure is high enough and/or it's not cold enough for this change to occur.