How to use "on second thought"

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"On second thought" is an idiom and it means a reconsideration of a choice, of a decision. It´s "used when you want to change something that you have just said, often to say the opposite"

"On second thought, I don't think I'll go."

It´s important to say that "on second thought(s)" might be also a noun and it means "a revised opinion or idea on a matter already considered". 

"He had second thoughts about his decision."