If a solid is irregularly shaped, we can determine its volume by measuring the volume of water displaced by the solid. For example, say you want to measure the volume of the toy dinosaur in Figure below . After placing the dinosaur in the water, the volume measured in the container increases by an amount that is equal to the total volume of the dinosaur. Note that this method only works for solids that do not dissolve in water. If you tried to measure the volume occupied by a pile of salt, the salt would dissolve in the water and this method would not work very well.
Beth is set up her new aquarium last night. First she  filled the tank with water leaving room at the top in case the water level to rose. Next she added gravel to the tank. She noticed that the water level rose 2 inches. After the gravel had settled she started adding decorations. She started with a large castle. Adding the castle took the water level up another inch. She looked at it a minute and decided she did not like it there. Beth lifted the castle out of the water and replaced it to one side of the tank. She finished by adding some small plants and a tiny treasure chest.
The gravel had a lower volume than what?