Brownian motion
When you push the start button the 0.5 micron diameter red bead will start Brownian motion (diffusing) and will eventually escape the circle, which is 5 microns in diameter. Pushing the repeat button will start another experiment, but the movement of the bead will be different, and thus it may take more or less time to get to the circle. The average time needed to escape can be calculated using Einstein's equations.