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The spell Animate Objects states:

Choose up to ten nonmagical Objects within range that are not being worn or carried. Medium Targets count as two Objects, Large Targets count as four Objects, Huge Targets count as eight Objects. You can't animate any object larger than Huge. Each target animates and becomes a creature under your control until the spell ends or until reduced to 0 Hit Points.

The spell description doesn't mention the objects needing to be solid, so I was wondering if there are limitations with regards to animating gases or liquids.
Could you choose to animate a cloud of fog or smoke? A puddle? A small pond? Perhaps the contents of a vial of acid?

Nope.

From the DMG at "Objects" (p.246):

For the purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone....

Your gases and liquids aren't "discrete items" in any natural-language sense of the word, so they're not (game) objects and aren't valid targets for animate objects.

Yes. Kind of.

The Objects section cited on pg 246 by @nitsua60 says:

For the purpose of these rules, an object is a discrete, inanimate
item like a window, door, sword, book, table, chair, or stone...

But discrete, according to Dictionary.com:

Discrete:

adjective

apart or detached from others; separate; distinct:

...

consisting of or characterized by distinct or individual parts; discontinuous.

Clearly, a puddle or cloud of vapor or gas can be considered 'discrete' by the dictionary definition (DnD does not provide a special definition of a 'discrete' object, however a quantity of water from a larger pond or lake, or a part of a cloud or a fog bank could not be considered discrete. Nor could one cobblestone in the road, unless you dug it out.

So sure, you could Animate any of these things, but nothing about the spell gives them solidity. Your gas cloud couldn't hold itself together, your puddle wouldn't have any integrity to keep it from drying out or being splashed apart. Your DM might rule that such a creature would have 0 hit points from the outset, and the spell would fail immediately. However, animating a nifty water or cloud creature could be a Rule of Cool thing, so give it a try.