Why are the surfaces of liquids always perpendicular to the gravitational force? I am not a physicist nor do I have a good knowledge of the topic. Pardon me if I use terms erroneously. 
I observed that when I put some water in a bottle, without regards to how the bottle is placed, the surface of water is always perpendicular to the direction of gravitational force. What is the explanation behind this?
 A: I think you mean horizontal to the direction of gravity. The answer is simple. Any part of the water surface which rises above the general level is dragged down by gravity until pressure from the rest of the liquid prevents it from sinking any lower.
A: To add to other answers - you could also say that the water will, in most cases, not be perfectly level. In a smooth sided water bottle, for example, if you look very closely, you will see a raised curved edge where the water meets the bottle. This is called a 'water meniscus', and is to do with the surface tension properties of water. In small containers such as test tubes, the effect is very strong, and the surface of the water is in fact curved instead of level. In terms of forces, it is the electromagnetic forces of the materials overcoming the force of gravity.
A: It is the same reason that water will run downhill.
If the water in the bottle is not level, then the water that is higher will "run downhill" until the water is level.
A: The math shows that when the liquid is level, it minimizes the total gravitational potential energy of the liquid. I know that in actuality, Earth is spinning and in Earth's frame of reference, the real natural tendency is for the surface to become perpendicular to the vector sum of the gravitational force and the centrifugal force. I know there is no such thing as potential energy of a fictitous force. However, when matter, gets further away from the axis of rotation, it causes Earth's rotation to slow down and have less rotational kinetic energy. I believe it can be show that when the surface is perpendicular to the vector sum of the gravitational force and the centrifugal force, it minimizes the sum of the gravitational potential energy of the liquid and the total kinetic energy of rotation. For this reason, we could define potential energy of a fictitous force. Then you would have to be careful to realize that it's no longer true that total energy is conserved according to this definition and it's just true that the total mechanical energy of the liquid is conserved in the frame of reference of Earth.
I don't believe potential energy or kinetic energy are real things that physically exist. They're just a complex mathematical property of the system we defined because once you define them, you can show that the laws end up conserving the total of them and use that to simplify calculations.
A: This is because liquids cannot withstand shear stress or tangential stress ( solids can). Imagine the bottle to be cylindrical and assume that the cylindrical liquid column inside is a solid(hypothetical) .  Now as I tilt the bottle, the column also gets tilted as it is tightly fitted and it withstands the shear stress but liquid on the other side( if the bottle is tilted ) will regain its previous position (when the bottle was not tilted )as it flows  i.e it will become perpendicular to the direction of  gravity (direction : vertically downwards) .
Note - Above, I assumed that the bottle is not accelerating .
There might be cases when the bottle is accelerating (say horizontally) then the water surface will be perpendicular to the effective force of gravity (which is ofcourse not vertically downwards in case of accelerating containers).
A: the reason why liquids assume level surfaces is that liquids cannot sustain shear stresses the way that solids can. Their inability to withstand shear means that in the presence of gravity 1) they assume the shape of whatever container they are placed in, and 2) the exposed surface becomes level with respect to the direction of gravity.
