# Come in or come into or both?

I haven´t had any problems with "in or into" as well as "in to" until now.

I know how to differentiate the three, but I can´t explain the following examples:

"You can put the milk in/into the fridge!"

"Do not let your skin come in/into contact with the acid!"

This is said to have the same meaning, therefore, one can use both. But why is it the same in meaning, really?

You can´t say "Come into" when asking someone to enter a room, can you?

X to Y means Y is the destination of X's motion.

X in Y means X is surrounded on most sides (or most of a single "side" in the case of a cylinder or sphere) by Y.

Into combines these meanings, X into Y means Y is the destination of X's motion, and that when the motion completes, X will be surrounded on most sides by Y.

You can put milk into the fridge

Anything that's a 3d "box" that contains items works with in. This is valid. Rooms are basically boxes, so it's valid for "Come into the room" as well.

Do not let your skin come into contact with the acid!

Liquids surround things on all sides when items are dunked in them. This is the implication, and it's valid.

You have several different cases.

In and into are both prepositions, with similar but not identical meanings: into implies (real or notional) movement, while in does not (but is not incompatible with movement).

So in your first example, they are really interchangeable, with no difference in meaning.

In the second case, I would say that there are two different idioms, with slightly different meanings. When the meaning is literal physical contact, come into contact with (skin) seems much more natural to me that come in contact with; but in a social sense come in contact with (people) seems just as good as come into contact with (people).

In the third case, "Come in!" in is not functioning as a preposition, but as an adverb. Into does not have that function.

• +1. We can think of contact as a state (being entered). Compare: come into estrus
– TimR
Commented Mar 20, 2018 at 16:18
• Come into is usually, er ... transitive? You have to come into somewhere. You could (though it's a bit odd) say come into the room, then. It is more idiomatic to say, say, come into the light [where I can see you]. Commented Mar 21, 2018 at 0:48