Today I passed by a restaurant and got attracted to its menu sticking on the window, so I stopped and had a look at it. Then a staff standing at the gate said to me,'We have nice food. Come on in!' I wonder why she said 'Come on in' rather than 'Come in'. I remember when I was partying with a group of friends and another friend passed by, I shouted to her, 'Interested? Come in!' Did I say anything wrong? Should I say 'Come on in'? And why?
"Come in" is permission, offered to someone who has asked for it (by knocking, for example). Unsolicited, it sounds imperative, or presumptuous; though of course this can be moderated by tone of voice or other context.
"Come on in" is an invitation, much better as an unsolicited offer to someone who may not have been intending to enter. Offered to someone who has presented themselves to your door, it rather conveys that you'd welcome them even if they hadn't asked to do so. And it suggests that they will be joining you in a shared experience, rather than just entering a space for which you are the gatekeeper.
"Come on in" has the same meaning as "come in" but is a more folksy way of extending the invitation. It suggests a kind of rural, down-home hospitality that is redolent of (American) TV shows of the '50s, which were ever a myth (although a persistent one) about how friendly people in the hinterland were.
This is an AmE usage, so I wouldn't expect to encounter it in the same way in Britain, though it may be available in some of the many dialects there.
For those who can't get past my suggestion that this is a "folksy" expression, note that I said it is "redolent of (American) TV shows of the '50s," not something like "absolutely 100% a hinterland expression."
Also, the fact that it can be used in Britain proves nothing except that the English language is flexible enough to express an idea in multiple ways. The fact remains, the expression is something heard quite a lot in the US, and carries with it enough regional overtones to be describable in that way.
It is never easy to answer why a particular colloquial phrase is used. It just is. In this case I speculate that "come in" on its own might be thought to be slightly less encouraging than "come on in".
You say "come in" if someone knocks on your door, and in that context it means "you may come in (if you really want to)". Whereas if you want to encourage someone to do something you might well say "Come on!" So "come on in" means "do come in. You are welcome".
But since these are spoken words, you can achieve the same welcoming effect with "come in" on its own by the way you say it.
"Come on" is a phrase of its own used to encourage/invite. (And express exasperation, but that's not a relevant usage to this conversation).
"Come in" is very similar phrase but it used exclusively for entering.
"Come on in" combines the two phrases in order to emphasize the message that the ones being adressed are welcome.
"Come on in" is like a welcoming and 'colorful' way of saying "come in". It implies a continuation of what you were doing before. So when someone says "come on" you might say they are in essence saying "continue with what you/we were doing in coming into the " and as others have suggested it is meant as a warm and friendly means of greeting and welcoming a visitor.
I will make note of the fact that it reminds me of other similar phrases that use "on" to imply continuation. "You go on right ahead" and a negative "Get on with you." With this it reminds me of the Spanish "adelante" which people would say to me at the doors of large office buildings in Santiago Chile where I taught English privately. This is simply another languages "continue on" + "going in" for lack of a better way of putting it.
These are like any other colorful expression in that one must use them in the correct moments or else it's possible to sound awkward or foreign as it is usually very obvious to a native speaker that someone is not a native speaker in that they fail more than not in knowing how to use colloquial expressions or slang. You should learn these in the context they're used rather than in a book or online. If you really want to get good with this, I suggest going downtown where there will be a lot of native speakers and pretend to read a book but instead just listen to as much native speakers conversing as you can. Good luck and keep asking questions!
First post here btw. <3
Having been raised in a place where this is more common that the simple 'come in' I think you have to first consider the idiomatic phrase 'come on' first and think of it as an extension or modification of that. 'Come on' in this context has the primary implication of a minimal urgency or impatience. For example:
- "Come on, let's go" when your kid is throwing a tantrum in the store.
- "Come on, are you kidding me?" When you have to wait at the same traffic signal through two red light cycles.
So "come on in" has a slight sense of more urgency to it. It kind of suggests "why aren't you already inside?" You might also hear "come on in already". The best way I can explain this kind of phrasing is a sort of ironic pleasantry.
Come on is an imperative form suggesting an invitation or exhortation. (Cf. for instance how it appears in the lyrics to Little Eva’s “Loco-Motion”.)
From the Oxford English Dictionary, to come on, 1. b.
imper. Used to urge a person (or animal) to advance towards or accompany the speaker, or (more generally) to continue or proceed with some action or activity.
With in, down, etc., that gives us the related 1. a. (b):
With adverb of direction, e.g. down, in, round. To move or travel onward in the specified direction. orig. and chiefly imper., as an exhortation or invitation, esp. to join someone or to enter or visit a house, room, etc.
Come in, by itself, suggests giving permission to somebody who wanted to come in anyway. It’s what you say when someone knocks at your door. Come on in, though, is an explicit invitation to people who might not have thought of coming in, or might not be sure they want to. It’s what you say when you’re standing outside a shop trying to get people to come in and see what you have for sale. (Or, as in your case, when you’re trying to get someone to patronize your restaurant.) And it’s what you say to someone who knocks at your door but seems hesitant or diffident about actually coming in.
Once you walk through the doorway, you have "come in". But the the focus is not coming into the physical building, but to participate in whatever's happening. The phrase "come on in" has been adopted to refer a more abstract idea of "coming in" to whatever's going on, rather than just coming into the physical location.