If action and reaction are opposite, then why don't cancel?

Physics: If action and reaction are opposite, then why don't they cancel each other?

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Well, action and reaction forces are equal and opposite but they do not cancel since they are not applied on the same body. 
For instance, 

if a block of mass m is sitting on a table. The action force 'mg' is applied by block on the table while the reaction force 'N' is applied by the table on the block in the form of Normal reaction(same magnitude 'mg'),hence they do not cancel.

Edit : I feel the need to explain the need of reaction force, for the above example, had the normal reaction not been there, there would be no force stopping the' m' mass block from going down, so its because of the normal reaction force that the block doesn't tear down the table and move downwards since the force acting on the block is cancelled.