Why don't water-controlling spells work on the water in pipes?

Question

I recently asked this related question but some things are still not clear to me. Why don't spells like Control Water, Shape Water, and Create or Destroy Water (and possibly others?) work with water in pipes (let's say about a foot in diameter if that matters)? Also let's assume we can see into the pipes (i.e. they are made of glass and/or they are open on one end.

Mainly I'm interested in explanation why doesn't water in pipes fulfill the definition of "freestanding water", "an area of water" or "an open container". Maybe it's just a linguistic question and it's not clear to me because English is not my native language but I'd really appreciate some guidance on this.

• Control Water: I am aware of What the heck is "freestanding water"? but "definition of freestanding: not relying on or linked to anything else; independent" doesn't make much sense to me because a lake or a river is relying on the banks the same way as water in a barrel relies on the barrel, isn't it?
• Shape Water: Why is inside of the pipes not an area of water?
• Create or Destroy Water: Why is pipe not a container? The open pipe is definitely open and even the closed one has an opening somewhere if the water keeps flowing - isn't it just a really long container?

Answer 1

It's not about pipes

As the answer to your linked question states,

By default, spells do not work without clear line of sight. Therefore, a solid pipe blocks spells, regardless of the construction material

So, you cannot target the water in those pipes, unless the pipe has an open end or faucet, where it is openly accessible. Not because the game cares about pipes, but because that is how the spellcasting rules work.

Whatever the exact definitions of "freestanding", "an area of water" or "an open container" amount to, while we do not know the reasons why the designers wrote the spells this way, we can guess that it is likely to avoid abuse against living beings. As about 60% of the human body is made of water, you otherwise would get into all kinds of discussion if you can control an opponent's body, by controlling the water it is made up from, or if you can kill an opponent, by destroying the water in their body. This obviously is not what those spells are designed to do.

That this may have a collateral effect on water in pipes, even if the pipe has an open end, is likely just that, an unintended side effect. As all these terms are no further defined, what it will come down to in the end is how your DM interprets those terms.

• What freestanding water is discussed in the linked question. Either, water not chemically bonded, or water in a pool, river, lake etc, water not held in a structure. As control water also talks about an area, my take it means lakes and the like, but your DM may have a different read.

• A container is defined by Oxford Languages as a portable compartment, so a pipe would not be a good fit for it, unless it was detached and could be carried around. But of course, the pipes do contain water, so if it is open at one end, the DM might decide you can affect water that you can target, and stretch the definition of container to allow this.

a : a receptacle (such as a box or jar) for holding goods
b : a portable compartment in which freight is placed (as on a train or ship) for convenience of movement

• What is an area of water? The term area is likewise not further defined by the rules (although they do define what Areas of Effect are). Again according to Oxford languages:

the surface included within a set of lines; specifically: the number of unit squares equal in measure to the surface

The spell says

You choose an area of water that you can see within range and that fits within a 5-foot cube.

which means you need to be able to see the water, and due to spellcasting rules, and you need line of effect to target it. The term area by the dictionary definition here does not really make a lot of sense, because you of course do not target and affect only the 2-dimensional surface of the water, you affect the actual 3-dimensional volume. Again, if your DM decides that you can target the water because there is an exposed end to the pipe, and if the pipe is transparent so you can see the water, you might be able to affect the water within the pipes that fits within a 5-foot cube.

Answer 2

Open to abuse

The obvious problem with not putting tight constraints on the "type" of water that can be affected is that the human body has an awful lot of water in it - for example, most blood plasma is water, urine is water with various extras, perspiration... The same applies for most other real-world living organisms and can be assumed to apply to most fantasy races and creatures.

Now have a look at the various options just in Control Water and think about how these would apply if the "water" being affected was that in the creatures in a cube 100 feet on a side. As for Create or Destroy Water - if all the water in a creature's body (or just a vital part, such as the head) is destroyed then any knowledge of anatomy will tell you that is an instant kill from a 1st level spell, no matter how many hitpoints the target has.

To avoid these sort of abuses even (or especially) when dealing with enormous creatures that may have equally enormous blood vessels / stomachs / bladders etc I suspect that the writers of the rules have simply taken a brute force approach to preventing game-balance-breaking shenanigans. However, do not forget Rule Zero - if you are attempting to use a spell in a creative-but-not-unbalancing way and can make a good argument to your GM then you may get what you want.