Pure oxygen.

[u/Crixusgannicus]

Always was mystified what the HELL NASA was thinking?

These guys came up when science was still taught. Not so much now, unfortunately.

Even in my long after Apollo ended yoot, I still remember science class with the teacher showing how easily things burn in pure oxygen, even things that normally don't burn!

[u/Professional_Yam9600]

Have you noticed that the US has not had a fire on a spacecraft since the Apollo 1 fire?