Explanation:
The setup contains rock, cylinder, covering lid and atmosphere. One or a few may contribute to the loss of water.
They may be dry, so it might have absorbed water. The rock may be porous and absorb water due to gaps between its molecules.
The material used as lid to cover the cylinder may be not good enogh to stop water from evaporation. The cylinder you might have kept near a hot source and water may have converted water vapor.
