Come on means 'hurry up!', 'you can do it!' or 'oh, really?'. You say it to hurry someone, to encourage them or to show that you don't believe them. About lights, heating and machines, come on means to start working: The lights came on. The exclamation is informal: it doesn't belong in an essay, but it sounds natural in Speaking and in everyday life.
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Come on! (hurry up / try harder); Come on, + sentence; Oh, come on! (disbelief); the lights/heating + come on
- Come on, we'll miss the train if you don't hurry.
- Come on, just one more kilometre and then we can have a rest.
- The heating comes on automatically at six, so the flat is warm when I get up.
What changed
First versionSomeone knocked on the door, and I called out, 'Come on!'
RevisionSomeone knocked on the door, and I called out, 'Come in!'
When someone knocks on the door, you answer Come in!, which invites them to enter. Come on! on its own is not an invitation to come in: it hurries or encourages someone. For a warm 'come on, come in', people say Come on in!
The meaning depends a lot on intonation. A cheerful Come on, you can do it! is supportive. A drawn-out Oh, come on! shows disbelief or irritation: Oh, come on, you don't really believe that.
Your turn
We're already late! ___, put your shoes on and let's go.
Show the answer
Come on — Everyone is late, and the speaker wants someone to put their shoes on and leave, so Come on, meaning 'hurry up'. Come in invites someone to enter, which doesn't fit here: they are leaving the house.
Imagine you're encouraging a friend before an exam or a job interview. Start your sentence with come on and give a reason why they will do well.
Come on, you've prepared for this interview for two weeks, so you'll be fine.
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