@The-Fall... What year are you in? XD I think your in a mod.

no, Its soon to be added to the official version.

https://github.com/FacialTurd/The-Powder-Toy/commit/03fde95719df5c917de84a8a791bd3f42e3c7859

See the github Commits.

Its also already in Jacob1's mod in which he operates off the newest source.

What's the deal with "There will be no movable solids."

What is a movable solid? And why can't it be moved?