Science: Cartesian Diver

              Cartesian divers are like eye droppers, and people can make fun little "submarines" with them.
              When you first fill the droppers up to about half way and plump it into the bottle, it floats around, and this is called positive buoyancy. Positive buoyancy is when the density, which is calculated by dividing the mass of the object by its volume, is less than the density of water, which means the density must be less than 1. 
              When you squeeze the bottle, more water flows into the dropper, creating a ballad, or a weight that weighs the object down. Because of the new amount of water in the dropper, its mass changes, and slowly, the cartesian divers sink, and the positive buoyancy turns into negative buoyancy, the state where the object is sinking. During negative buoyancy, the density of the object is more than 1.
             There are times when you squeeze the bottle just enough so the dropper hobbles around in the middle of the bottle. This is called neutral buoyancy. During neutral buoyancy the density of the object is exactly 1. 
              When you let go of the bottle, the water forced into the cartesian diver backtracks and comes out, changing the mass of it once again - to something lighter. Then, because of this big change, the dropper floats up

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