"Fluids" versus "liquids"?
Solution 1:
A fluid can either be a liquid or a gas; as such, fluid is a hypernym of liquid in this regard. In your example
Drink plenty of fluids
it's only idiomatic to use fluids instead of liquids. (I wonder what else could be drunk besides a liquid/fluid, anyway!) You also hear things like:
Your body needs lots of fluids
In day-to-day usage, one will find liquid only used when the state of matter of a substance is in question or important to note. For example:
"How's the brownie mix looking?" "It's still basically liquid!"
When ice-cream is involved, melted is more likely to be used instead of liquid. One may also find liquid used in a financial context:
They think I'm liquid but, man, I'm flat broke!
Fluid just sounds smoother and it rolls off the tongue much easier. That probably explains why it's more commonly used in place of liquid. People would probably use gas if they wanted to be specific. Here are some common idiomatic phrases:
- body fluids
- engine fluid
- transmission fluid
- poisonous liquid
- hot liquid
- toxic fluid[s]
- liquid coolant
- cooling fluid
From the above examples, you may notice that fluid tends to be used for a liquid substance that has a specified/particular purpose. Thus, transmission liquid would be unidiomatic, for instance.
Solution 2:
A liquid is a state of matter in which the molecules move freely but tend not to separate; e.g. water at room temperature. A fluid is simply something that flows, be it gas, or liquid.
Solution 3:
Fluid describes a substance that has no fixed shape and changes shape freely based on outside influences. It can describe two states of matter: gas and liquid. To think of it another way, a liquid is a fluid, but a fluid is not necessarily a liquid.
Solution 4:
In general usage, "liquid" and "fluid" are synonyms.
In a scientific context they are not.
"Liquid" is a state of matter: solid, liquid, gas, plasma, Bose-Einstein condensate
"Fluid" describes certain properties generally attributed to substances in a liquid state
From Wikipedia
Fluids display such properties as:
* not resisting deformation, or resisting it only lightly (viscosity), and
* the ability to flow (also described as the ability to take on the shape of the container).
The degree of fluidity depends on the substance and the prevailing conditions (e.g. temperature and pressure).
Consider water, motor oil and maple syrup. At room temperature, they are all liquids. As the temperature drops, all three liquids get denser and flow more slowly.
At 32° F / 0° C water becomes a solid (ice) and no longer flows. Motor oil and maple syrup continue to flow, but the syrup flows more slowly.
In general usage it has become "less fluid".
Solution 5:
As a noun, Merriam-Webster's explicitly defines liquid as "a fluid."
a fluid (as water) that has no independent shape but has a definite volume and does not expand indefinitely and that is only slightly compressible
Unless you are writing in a scientific or explicitly academic context, I'd say the two are interchangeable. Note that as adjectives, they do have slightly different meanings.