MasterAdmin wrote:
Is the grammar correct in this dialogue?
A: That's good ice cream!
B: Yes, I put it into coffee.
paulb wrote:
If your question is about "in" versus "into", it is a little tricky. "In" is used much more for this type of thing.
The grammar is correct whether you use "in" or "into". I agree with paulb that "in" is used more often in this case, because this dialogue is informal. Encarta has an interesting Word Usage discussion on this:
in, into, or in to?

In formal written English, the preposition for inward movement is into, not in: She came into [not in] the room. We welcomed him into [not in] the family. It is sometimes acceptable to use either in or into, but the latter is usually preferable in formal English: He put it into [or in] his pocket. Using in for into can be misleading, as in She jumped in the pool. (Did she jump into the pool, or was she already standing in the pool when she jumped?) Do not confuse into with in to - the preposition into is never written as two separate words; when the separate words in and to occur side by side, they should not be joined together: I went into [not in to] the house. I went in to [not into] get my jacket.
Encarta(R) World English Dictionary [North American Edition] (2007) http://encarta.msn.com/dictionary_/into.html