When you edit, you are often creating
an deleting segments and nodes, even when you don't realize it.
If you take a moment to select the segments and nodes and look at their IDs in the left pane, you can see what is happening.
Here is what happened during an actual editing session.
[[File:Segment_node_ids1.png| thumb|Every segment is attached to two nodes - one node at each end of the segment. ]] [[File:Segment_node_ids2.png| thumb|At the junction, two segments are connected to the same node. There is exatly one node at a junction. ]] [[File:Segment_node_ids3.png| thumb|When a segment is highlighted and one end is dragged away from a node, A new node is automatically created on the segment that was just moved. ]] [[File:Segment_node_ids2.png| thumb|When a node is dragged and connected to another node, the "stationary" node (the one that didn't get dragged) is retained. The node that was moved is deleted. ]] [[File:Segment_node_ids4.png| thumb|When a segment is cut by connecting another segment somewhere in the middle ,two new segments are created. The node from the end of the segment used for cutting becomes the node for the new junction ]] [[File:Segment_node_ids5.png| thumb|A segment is disconnected from the junction and gains a new node at the end ]] [[File:Segment_node_ids6.png| thumb|Another segment is disconnected from the junction ]] [[File:Segment_node_ids7.png| thumb|Two segments are joined. The node from the stationary segment is retained as the junction node and the node from the moving end of the other segment is deleted ]] [[File:Segment_node_ids8.png| thumb|If you select the junction of 2 segments and drag it to another node, both segments will have their ends dragged along with the junction ]] [[File:Segment_node_ids9.png| thumb|All the segments are now joined at one junction. The stationary node is retained for the junction node. The node that was moved is deleted. ]]