I have wondered for a while now about this - and it might not be entirely possible in reality but bare with me here
A cube of 5cm3, filled up with 4cm3 of water.
The material of the cube is indestructible, doesn't give way and it's 100% waterproof.

The cube shrinks and eventually fills up entirely with water. The cube keeps shrinking. It won't stop getting smaller.
What happens next?