Hongmeng purple energy?
Turbidity?
Hydrogen?
Air?
Or...
There is a lot of air floating between heaven and earth, but there is a kind of air that is invisible and stateless. It cannot be seen, touched, or breathed, but it can be sensed.
Therefore, the pig's feet of this book are this lump of air that cannot be seen, touched or breathed.
Mission: Positive, patriotic and patriotic