An irregular solid can be measured using a technique called water displacement. When a solid object is placed in water, the amount of water that is displaced by the object is equal to the volume of the object. 

For example, we can determine the volume of a small rock as follows:

1. Fill a graduated cylinder with 20 mL of water. We'll call this the initial volume of the water.
2. Carefully add the rock to the water in the graduated cylinder. Whenever you place an object in a graduated cylinder containing water, the volume level of the water goes up. Let's say that after we added the rock, the volume of the water increased to 27.5 mL. We'll call this the final volume of the water.

The volume of the rock is equal to:

        final volume of water - initial volume of water

Therefore, 

        27.5 mL water - 20 mL water = 7.5 mL

The rock has a volume of 7.5 mL.