Rewind Button: The Rewind button is only accessible during Stop mode. Clicking on Rewind rewinds the simulation back to the last time you clicked Run, i.e., you can rewind the simulation to the last paused or stopped state. The indicated time on the clock changes accordingly.

The number of times you can rewind is equal to the total number of times you paused or stopped a simulation since you started up the simulator, minus the number of times you clicked Rewind.

For example, suppose you have run two simulations. You stopped the first simulation using the Stop button. The second simulation stopped when the clock reached the Stop Time. Between the two simulations, you clicked the Pause button four times. You have already clicked Rewind once. Thus, you can rewind (2 stops + 4 pauses - 1 rewind =) five more times.

Keep in mind the fact that Rewind only undoes the the changes in forces and speeds of bodies during simulation runs. It does not undo any other changes YOU make as the user., such as adding elements to the simulator, or deleting them from the simulator.