The midpoint of a segment is an important concept to master. The midpoint is the point that is the same distance from either endpoint of a segment. This applet illustrates that no matter where your endpoints are the midpoint is always exactly the same distance for each end.
The blue points A and B are dragable, that is you can move them with your mouse. You will note the distance AC is always the same as the distance BC.

This applet can be used by teachers in a demonstration mode in the classroom or teachers can have students load it on their own computers as a worksheet to be completed for a grade.