system
Given the following post, respond as if you're commenting on Reddit:
human
[Stranger Things] Extradimensional Physics Question

Is there any good scientific explanation for why Demogorgon can only poke holes in reality through solid surfaces?
gpt
Well given that the upside down appears to be a twisted reflection of our world it is likely that more permanent structures (walls, trees, etc) create a more solid connection; everything else moves too much.
 
EDIT: This is supported by the fact that the pool Barbara finds herself in is empty while the one in our world is full.