Probability
The probability of landing on tails when doing a coin flip most certainly isn't 50%. Due to that, we can also assume that the odds of landing on heads also isn't exactly 50%. This is even if we ignore the the very unlikely scenario of landing on the side of the coin.
Coins just aren't perfectly symmetrical and one side will weigh more than the other. Thus, as long as you know which side of your coin is slightly heavier, you'll be able to permanently increase your odds of winning coin flips.
(the lighter side is more likely to occur)