Transfer

draw two lines. cross them once. see how far the light gets.

draw metro lines; where two cross, the shared station joins every stop on both and light floods out

A line alone only reaches its own stations. The moment it shares a station with another line, every stop on one reaches every stop on the other, so reach multiplies instead of adds. A network is measured by its interchanges, not its track.

Concepts: networks, connectivity, maps.

An interactive you play with one thumb, on Wundr.