Water displacement was discovered by the Greek thinker Archimedes in the 3rd century BC. He worked out that if you put an object in water, the amount of water displaced (thrown out) is equal to the volume of that object. By measuring the overflow of water, you can work out the volume of the object (it only really works if the object is non-porous/ can't soak up the water. In fluid mechanics, displacement occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid is displaced can then be measured.