Step 5: Introduction to electronics and soldering
Time needed to build the circuit: 20-30 minutes.
After the circuit is completed, the concept of an electric circuit can be demonstrated with a "human electrical circuit". See the video of "Blinky the Led pet" for a demonstration.
The use of a led can also be explained with this circuit. Chances are that at least one of the students mounts the led in the wrong direction, which is my cue explaining the working of led's :-). The same goes for the transistor, of course...