A circle object, for example, has a data field, radius, which is the property that characterizes a circle. One behavior of a circle is that its area can be computed using the method getArea().

1)To me, area is a property/characteristic, not a behavior
2)getting the area is a behavior, but it`s not a behavior of the circle.

So, why is the getArea() method, considered a behavior of the object "circle"?

Circles are easy to work out the fields of; in 2-D they can never have more than 3 fields. Radius, x-position and y-position (of the centre). The area is derivative of the radius and is therefore calculated from it.