Everything in geometry is built up from simple points. In dynamic geometry, a point can be dragged from its current position to any other location. For instance, a line segment is made up of all the points (the “locus”) along the shortest (direct, straight) path between two points (the endpoints of the segment).
A circle is all the points (“circumference,” “locus”) that are a certain distance (“radius”) from one point (“center”). Therefore, any line segment from the center point of a circle to its circumference is a radius of the circle and is necessarily the same length as every other radius of that circle. Even if you drag the circle and change its size and the length of its radius, every radius will again be the same length as every other radius of that circle.
In this activity, create some basic dynamic-geometry objects and drag them to observe their behavior. Do not forget that you have to press the “Take Control” button to take actions in GeoGebra.