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When fluids are heated from the bottom, the regions of the fluid that get heated first expand and become less dense compared to the cooler, denser regions. This density difference causes the heated fluid to rise while the cooler fluid sinks to replace it.
Step 1: Heating causes the molecules in the fluid to move faster, thus causing expansion leading to lower density.
Step 2: The less dense (heated) fluid rises due to buoyancy, creating a convection current as cooler (denser) fluid moves down to take its place.
Therefore, the correct explanation is that heated fluid becomes less dense than the cold fluid above it.