When you balance a pencil on your finger, I have always wondered, "Why does the heavier part go down, and the lighter, up?"
I'm assuming you haven't taken any physics courses, so let's start by explaining the concept of a force. Forces are the central focus of classical mechanics. Basically, a force is a push or pull on an object as a result of its interaction with another object. When applied to an object with mass, a force causes the object's velocity to change in some way. Gravity is just the force of two masses attracting each other--in this case, the earth and your balanced pencil. The heavier side of the pencil has more mass, so the force of gravity on that side is stronger, causing it to "go down". There's actually a better explanation of this using rotational mechanics, but this should help!
The mass is distributed in the beam (pencil). The gravity pull the beam (pencil) at each side of the lever down. As consequence of the coherence of (beam) pencil and the presence of the hinge (your finger), the more heaviest side "wins" and the other side "lose".
In a tug of war, why does the stronger side, pulling with more force, win?
Both sides are pulling against each other, but in the end one always finds itself being dragged along.
It's the same principle here - each side of the pencil is being pulled towards the earth. Both sides would like to fall, but your finger prevents them - it plays the role of the rope in the tug of war.
There can be only one winner, and the heavier side, which is more strongly attracted to the earth (is pulled with more force), will win.
The force is greater, it's as simple as that.
(Though of course, it's never really as simple as that. Why does your pencil rotate? And how does the downward force on the heavier side lead to an upwards force on the lighter side? The mechanics of rigid bodies like your pencil can get very complex very quickly.)