This applet allows the user to simulate a race where the results are based on the roll of a die. The user can determine which player moves forward for a given roll, and can then experiment with the race by determining which player will win more often based on the rules that they specify. When a number is rolled, the winner moves the number of spaces shown on the die, meaning that the probability of a player winning the race is determined not just by how many of the rolls are assigned to him/her, but also by which rolls are assigned.
The rolling of a die is one example of probability. Each roll of the die gives you a one in six chance that a certain number will appear. Probability is just the expected value of what the outcome should be. If you run the applet a hundred times you will see that the probability you came up with (known as the experimental probability) will be close but not always exactly what you would expect (known as the theoretical probability).
The original version of this activity is part of the Shodor Education Foundation's Middle School Math Courseware, Project Interactivate, which contains other interactived activities as well as suggested lesson plans and supporting materials.
Original Applet: http://www.shodor.org/interactivate/activities/RacingGameWithOneDie/