A wire is not a simple thing: it is not a node. A wire is a physical object that has properties. For example, in power transmission lines the resistance of the wire is very important. When we work at very high frequencies, the capacitance of nearby wires to each other and even the inductance of the wire and the mutual inductances between wires can be important. We call these unwanted interactions "parasitics". They break our lumped-circuit abstraction, so we need to know just how bad they are