The Drag Test
When you construct a point to be on a line (or on a segment, or ray, or circle) in dynamic geometry, it is constrained to stay on that line; its location is dependent upon the location of that line, which can be dragged to a new location.
Use the “drag test” to check if a point really is constrained to the line: select the Move tool (the first tool in the Tool Bar, with the arrow icon), click on your point and try to drag it; see if it stays on the line. Drag an endpoint of the line. Drag the whole line. What happens to the point?
The drag test is one of the most important things to do in dynamic geometry. Make sure that everyone in your team can drag objects around.