Fluid vs. Liquid

yeah i know this is kind of geeky of me to post stuff to do with my homework, but it helps.

ongman dictionary defines liquid (NOUN or adj) as:

[uncountable and countable] a substance that is not a solid or a gas, for example water or milk:
Add a little more liquid to the sauce.

Fluid (NOUN or adj):

[uncountable and countable] technical
a liquid :
He is not allowed solid food yet, only fluids.
a powerful cleaning fluid.

So the question is how would you explain the difference? Is it merely an issue of technical term versus normal usage term? I need more lucid explanation for the two words. A student asked me the difference between them but I could not really discern the difference.

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