system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 What’s left after a brain tumours removed

When a brain tumour is removed , is there just an empty space left where it was? And does that space ‘fill up ‘ agin ???
gpt
Air. Apparently after surgery patients can hear the air bubble move around *inside their brain* as they change the orientation of their head. 

The air is absorbed gradually by the tissue and fluid in the brain, then removed via the lungs and exhaled. The lost volume of air is replaced by Cerebro-Spinal Fluid, the liquid which normally encases the spine and brain.