On second thoughts means 'actually, no; having thought again'. You change your mind on the spot and name the new decision straight away: I'll cycle there. On second thoughts, I'll take the bus because of the rain. British speakers say thoughts; Americans say on second thought, without s. Both are correct.
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on second thoughts (UK) / on second thought (US), + revised decision
- I'll cycle there. On second thoughts, I'll take the bus because of the rain.
- On second thought, could we meet outside the station?
- We planned to leave the door open, but on second thoughts we locked it.
What changed
First versionIn second thoughts, let's book the earlier train.
RevisionOn second thoughts, let's book the earlier train.
Nothing in the meaning tells you which preposition to use, so people guess: in second thoughts. But only on works: on second thoughts. With the right preposition, the phrase is the fixed one.
Have second thoughts means to doubt a decision you've already made: I'm having second thoughts about the trip. The person is wavering but hasn't changed their mind yet. On second thoughts is already the new decision: On second thoughts, let's stay at home.
Your turn
You reconsider a decision: ___ second thoughts, I'll stay for lunch.
Show the answer
On — The phrase starts only with On: on second thoughts. In second thoughts isn't said in Britain or America.
You suggested meeting at six, then remembered the traffic. Change your mind in one sentence with on second thoughts.
On second thoughts, let's meet at seven — the traffic is terrible at six.
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