The Sear Mechanism in a Firearm

Most firearms have a device called a sear mechanism in them. The fact that you don't really know when your finger is going to break friction with your thumb and start accelerating towards the fat of your hand, the sear is the same way. Imagine trying to slide, like you have a pencil and you're moving it off a table. When does it actually break free of contact and start moving? So that matters depending on if there was a little piece of dust in the sear mechanism between one shot and the next.