Spend a moment and watch the four cars move around the screen. It looks like three of the cars are following the red one.
In fact only one of the three apparently following cars is actively controlling for following the red sone; it has the intention or purpose of following the red car. The other two cars have no purpose; they are just conincidentally moving in the same path as the red car. Notice that it is impossible to tell, by looking at the behavior of the three following cars, which is intentionally following the red car. In order to tell which of the three cars is intentionally following ("tailing") the red one you have to introduce disturbances by changing the path of the red car. This is done my moving the mouse. The car that os intentionally following the red one will change its path when the red car changes its path becuase it is controlling for staying behind the red car.
In this demonstration you are using The Test for the Controlled Variable to determine which of the three following cars is intentionally following the red car. The following car that is controlling for following is behaving with the intention of following the red red and resists disturbances to that intended result. The demonstration shows that you can't tell whether a behavior (such as "following") is intentinoal (purposeful) without doing The Test to determine whether that behavior is under control.
Once you have detected the intentoinally follwoing car you can press the mouse butto and a new intentonally following car will be selected (it might be the same car as before). All cars will again line up behind the red car and you can try to determine which of the three cars is intentionally following the red one this time.
Notice that the movements of all three following cars appear quite lively; there is nothing that distinguishes the appearance of the intentinally following car from that of the unintentinoally (coincidentally) following ones. The only way to reliably distinguish intentional from unintentional behavior is by doing The Test to determine whether a variable (in this case, the pattern of movement of the cars around the screen) is under control
It should be very easy to tell which car is resisting your disturbances (mouse movements), especially if you use smooth, slow disturbances. Notice that only one of the following cars will be affected by your disturbances to the positoin of the red car -- the one that is intentoinally following it.