Conditional Probability and Independence
If you throw a fair die, with no clues given about the outcome, the chance of getting a six is 1/6. But maybe you have been told that this blue die, and another red die were thrown, and their total score was four. Then you could be sure that the score on the blue die was not six. Similarly, to be told that the total score was ten makes it more likely that the blue die scored six, as all scores lower than four are eliminated. Information about the total score on the two dice can change your original opinion about the chance of a six.