Minor illusion says:

If you create an image of an object - such as a chair, muddy footprints, or a small chest - it must be no larger than a 5-foot cube. The image can't create sound, light, smell, or any other sensory effect. Physical interaction with the object reveals it to be an illusion, because things can pass through it.

You could create an illusory fog cloud in a 5-foot cube. However, things pass through real fog clouds just as easily as through an illusion. So how exactly does "things passing through it" reveal an illusory fog cloud to be illusory?

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up vote 21 down vote accepted

Real fog still interacts visibly with things passing through it

If you pass your hand through a real cloud of fog, the fog will visibly flow around your hand, and the air current produced by your hand will cause the fog to swirl around after your hand passes through it. An inanimate illusory cloud of fog will not exhibit any of these effects, revealing it as fake. In addition, depending on what the fog is made of, it may have other associated sensory effects, such as dampness or coolness, none of which can be replicated by minor illusion.

(Also, as another answer points out, minor illusion in particular cannot produce the image of a cloud of fog.)

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    Also, fog is (usually) made of water. It usually has a cooler temperature to come into contact with it and can leave you slightly damp. – guildsbounty Nov 15 at 19:47
  • @guildsbounty That's a good point. I'll add that as well. – Ryan Thompson Nov 15 at 19:50
  • Fog machine fog works that way, but real fog? I'm not so sure about that. – Pink Sweetener Nov 15 at 19:54
  • @PinkSweetener A cloud of fog contained within a 5-foot cube is more like the product of a fog machine than ambient fog. – Ryan Thompson Nov 15 at 19:55
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    @PinkSweetener Real fog would work that way too: but real fog is usually surrounded by a much larger quantity of other real fog, that fills in the air when you displace fog by moving your hand through it. So it is usually not a visible event. However, a 5' cube of "fog" (if it could be created by this spell) would appear odd by not shifting with your movement. – Gandalfmeansme Nov 15 at 19:56

Fog Isn't An Object

As has been stated elsewhere, gasses and liquids are not considered objects. So fog would not be a valid image created by Minor Illusion.

There could be some edge cases where physical inspection might not reveal an illusion (for example, an illusion of a brick in a wall painted green, cast a nanometer in front of a real brick in a wall painted red, that would feel the same to a person investigating it), and a DM would have to decide whether to ignore that part of the spell's description. However, creating illusions of intangible things would normally be impossible with minor illusion.

NOTE: If the spell in question had been silent image (which contains similar text about physically interacting with the illusion, but allows the spell to create most any "visual phenomenon"), then see Ryan Thompson's answer on how physical interaction could still reveal illusory fog.

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    But a "bowl of water" works, so wouldn't a "5-foot cube box with walls of 'perfect' glass with fog in the middle" work as well then leaving the same problem anyway? – David Mulder Nov 16 at 12:16
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    Feel free to pitch that to your DM. My take on it would be that you haven't created "the image of an object" if the "object" part of your illusion is invisible. – Gandalfmeansme Nov 16 at 14:40
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    Also, according to the DMG 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, not a building or a vehicle that is composed of many other objects." The 5-foot cube box you described is an object (though it has no "image"), but as the mist is not contiguous within it, it's hard to argue that the box and mist combo is "discrete." (You could make it not a box but a strange undulating mass of glass, or a solid statue of foggy crystal: but in those cases it would be surprising that a hand passed through it). – Gandalfmeansme Nov 16 at 14:45
  • Not sure why this is not the accepted answer, but yeah, I came here just to check if anyone had already given this answer. Still it should be improved by adding Ryan's answer in order to be complete. – HellSaint Nov 17 at 5:14
  • I figure that Ryan's answer is already the best version of the explanation of what he was explaining. That's why I linked to his answer in my note at the end. – Gandalfmeansme Nov 17 at 5:21

In my experience of fog, you can see it a ways away but as you near fog, you realize that there is no point in time that the fog fully obfuscates your ability to see. So if you were to walk up to and into illusory fog, it would continue to block your sight. REAL fog doesn't.

So being aware that real fog doesn't block sight as well as fake fog would, you would realize that it is an illusion, and thus, see through it.

Pun intended.

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    This idea is somewhat complicated by the conjuration spell fog cloud, which creates fog that heavily obscures an area. If you ran into fog that obscured your sight, you might conclude it was actually there, but the result of a different spell. – Gandalfmeansme Nov 16 at 3:57

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