set off (phrasal verb)
1. to start a journey
We set off at six o’clock, and hiked through the mountains all day.
2. cause the start of, activate
He burned the dinner in the oven, and the smoke set off the alarm (set the alarm off).
3. enhance, show a nice contrast
The blue scarf set off her pale skin and blue eyes nicely.
4. explode, blow up
We always set off fireworks on the Fourth of July!
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