When an outcome is uncertain but entails a win or a loss, the simplest criterion for assessing it is the expected value: the mean of the winnings weighted by the probabilities. From it arises the idea of a fair game (zero expected value) and its concrete counterpart, the margin that betting houses secure by setting the odds. These examples show how to compute the expected value of a bet and how, conversely, to derive the odds to set in order to obtain a desired margin.