set off
With or without an object
Begin a journey
We set off at dawn.
Cause something to start or go off
The burnt toast set off the smoke alarm.
With or without an object
It can be used with no object, and it can also take one. With an object it is usually separable: turn the volume up, turn it up.
The particle: off
This verb belongs to the group ‘setting out and leaving’.
Practise it
Add it to your daily practice or see every verb with off.
Further reading: set off on Wiktionary (CC BY-SA).