Why Did the Water Level Drop in a Sealed Graduated Cylinder?

[cornflakes]

A rock is placed in a graduated cylinder containing water, and the cylinder is covered to avoid evaporation. The next morning the water level has dropped by 5.6 ml. No one has had access to the cylinder, and the cylinder has no leaks. What is the most likely explanation for what happened?



Thank you in advance

 

[rock.freak667]

The rock was very dry and it absorbed some water?