The density of an object is the ratio of its mass (weight) to its volume.
An object will float if its density is less than the density of the fluid. An object will sink if its density is greater than the density of the fluid. You can think about comparing the mass of the object to the mass of the same amount (volume) of the fluid.
For example, say we have three bottles that are the exact same size/volume: one is empty, the second is filled with water, and the third is filled with lead. The empty bottle will float in water because it weighs much less than the bottle filled with water (less mass in the same volume = lower density). The bottle filled with lead will sink in water because it weighs more than the bottle filled with water (more mass in the same volume = higher density). In science, we can not always compare the mass of objects that are the same volume - that is why we have the mass to volume ratio called "density".
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