Physical events are sudden changes of variable values. The change takes effect within one moment in time.
The above figure shows a physical system, where balls collide with other balls and with the walls of a box. The impacts are physical events. At the time of an impact the speeds change abruptly.
On the function plotter you can see, that some of the variables have points discontinuities or are least have points, where they are not differentiable. On the timeline beneath the plot ordinary frames are represented by short, grey ticks 
and events are represented by long, black ticks 
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