This question has been extremely irksome and I wish to be answered. Please see the figure below.
When a body is placed on a table, the force of gravity is pulling it down;but it cannot move down because the table top restricts its motion. In this case the two forces which we considered here are the force of gravity acting vertically downwards on the block and the reaction force offered by the table on the block top in the vertically upward direction along the same line. The forces are acting on the same body and cancel each other.
Here the forces are not acting at the same point, but are collinear – acting on the same line.
So, what is the misconception here?
You ( and many others) imagined that the body exerts a force on the table top, whereas it is the force of gravity acting on the body trying to bring it downwards. But, when to downward motion is restricted by the table top, the force of normal reaction comes into play.
(This is one way of explaining it. Any other answer visitors? Please post them as comment to this post)