trigger

A trigger is the part of a gun that you push with one finger when you want to make the gun shoot.

Example: I didn't want to make him angry, because he had his finger on the trigger.

A trigger is a little thing that makes something bigger happen.

Example: Bad smells are one of the triggers for my headaches.

A little thing triggers a big thing to happen when it causes it.

Example: Those angry words triggered the beginning of the war.