Explain how to determine the volume of an irregularly shaped object such as a rock

If you have no way to accurately measure all of the object's bumps and dimples, then the only way to measure its volume is by means of fluid displacement. 

-- Put some water into a graduated (marked) container, read the amount of water, drop the object into the container, and read the new volume in the container.  The volume of the object is the difference between the two readings.

-- Alternatively, stand an unmarked container in a large pan, and fill it to the brim. Slowly slowly lower the object into the unmarked container, while the pan catches the water that overflows from it.  When the object is completely down in the container, carefully remove the container from the pan, and measure the volume of the water in the pan.  It's equal to the volume of the object.