Density is a measure of how concentrated a substance is. i.e. how much matter is crammed into a given volume.

In this video Pepsi and Diet Pepsi cans are lowered into a water tank. The Pepsi is denser than water so sinks but Diet Pepsi is less dense than water (due to the artificial sweetener) so floats.

Likewise materials with a high density will sink in one of lower density.  An iron cannon ball will sink in a bowl of water because iron is denser than water. Mercury on the other hand is denser than iron so the cannon ball will float in a bowl of mercury

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Upthrust and Archimedes’ Principle

The following videos hopefully explain this phenomenon

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