Tk automatically creates class bindings for radio buttons that give them
the following default behavior:
The radio button activates whenever the mouse passes over it and deactivates
whenever the mouse leaves the radio button.
The radio button's relief is changed to sunken whenever mouse button 1 is
pressed over it, and the relief is restored to its original
value when button 1 is later released.
If mouse button 1 is pressed over the radio button and later released over
the radio button, the radio button is invoked (i.e. it is selected
and the command associated with the button is invoked,
if there is one). However, if the mouse is not
over the radio button when button 1 is released, then no invocation occurs.
The behavior of radio buttons can be changed by defining new bindings for
individual widgets or by redefining the class bindings.