Dolphins are mammals- so they have lungs, and breathe air through a 'blowhole' on top of their head. They do not have gills like the fishes.
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Being mammals - they lack the swim bladder that fish have. They simply use their tails to 'steer' themselves down or up.
Helium is light and it will float / rise.
Objects that are less dense than the fluid they are in will float, while objects that are more dense will sink. The buoyant force acting on an object is determined by the difference between the object's weight and the weight of the fluid it displaces.
Huge slabs of rock rise and sink due to movement in the Earth's tectonic plates. When plates collide, one may be forced below the other, leading to subduction and sinking. Conversely, when plates separate, magma rises to create new crust, causing rock to rise.
The cool air will sink and the warm air will rise.
The Earth is said to be undergoing differentiation when heavier elements sink towards the core and lighter elements rise to the top. This process results in the formation of distinct layers within the Earth based on differences in density.