Final answer:
Pulling the trigger on a firearm causes the firing pin to strike and explode the primer in the base of the cartridge or shot shell.
Step-by-step explanation:
When pulling the trigger, it causes the firing pin to strike and explode the primer in the base of the cartridge or shot shell. The firing pin is a small, metal pin that is pushed forward by a spring when the trigger is pulled. This impact creates a spark which ignites the gunpowder or propellant in the primer, resulting in the bullet being propelled out of the barrel.