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The fluid won't expand, but the vapors in the empty part of the can will.


Fluid expands when heated also. Remove your radiator cap while the engine is hot for a demonstration.


<img src="/forums/images/graemlins/shame.gif" alt="" /> That's not the fluid expanding.
By removing the cap you are allowing the fluid to become steam by introducing air in the system, which happens violently btw.
It is the steam that causes those major burns when people take the cap off a hot radiator.

He is correct, fluids expand much, much less than gasses.