When you flip a coin, the probability of it landing with EITHER side showing is 100%.
This leads us to the rule ...
The sum of the probabilities of all possible outcomes is 100%.
For a coin: (probability of heads) plus (probability of tails) = 100%.
That just says: We're 100% sure that the coin will land with either heads or tails up.
An "honest" coin gets heads 50% of the time and tails the other 50%.
But if the coin is all bent and squashed and has a feather stuck to one side and a wad of gum on the other side so that it comes up heads 70% of the time, then the coin isn't 'honest'. But it still has to land EITHER heads OR tails, so the sum of the probabilities is still 100%.
