Naturally people assume that the probability of flipping of coin and getting a head or tail is 1/2. Is it really???

The truth is we don't know. If either side is heavier, gravity will make it more likely for that side to face down. Hence the opposite side will be more likely to come upward.

The theoretical probability of getting a head should be 1/2, since both side should have the same weight. However only via experimentation of flipping can we deduce an interval of probabilities where the true value lies.