system
Given the following post, respond as if you're commenting on Reddit:
human
[D&D] How do Gelatinous Cubes maintain / hold their shape?

The title pretty much says it all. They are creatures made up of slime and instead of being a blob, they instead somehow maintain a cube. I could understand if they were somehow a metal slime or a mimic, but we are talking about what is basically just acidic jello.
gpt
Indeed, jello holds its shape due to collagen proteins that bind loosely together, creating a gel that holds the water and other ingredients in shape. 

Gelatinous cubes are living creatures with their own biology, their own cells even. They build collagen to hold themselves together and give enough rigidity for them to pull on themselves and wiggle to move. 

However, the collagen bonds are loose enough that they get easily broken or moved aside when a more solid object gets pushed inside.