I once read that Macintosh computers are designed on a Cray supercomputer while Mr Cray designs his supercomputers on a Macintosh. True or apocryphal, these days experimenting with circuits doesn't happen to the smell of solder. They've got programs, see.
Crocodile Clips is a very competent electronic circuit simulator. On one level, you build and test circuits on the screen exactly as you would with the real thing. While it is no substitute for that, you can still use a diverse bank of components from meters to motors and relays to resistors. There are transistors, capacitors, Zener diodes and light emitting diodes. There's even a couple of integrated circuits in the shape of a timer and an op-amp.
So using your mouse you drag and drop these on the screen. You wire them up as you draw, with a virtual reel of wire, from one point to another. Should you add a wrong wire, you simply use a "crocodile" tool to eat (or del-eat) it!
But on another level, this a great modelling tool. So when you click on a switch, a lamp will light up or a buzzer will sound and arrows show the direction of the current. If you vary the current, by using a slider control, the "light" and the "sound" level change too. If you had meters in the circuit you would see the readings change. And if that doesn't make things clear, natty bar voltmeters on each wire of the circuit give a good sense of just how much.
More than this, you can drop probes into the circuit and show the readings over time on an "oscilloscope". So you might play with the value of a capacitor, a sinusoidal power source and see the effect. The scope here, no pun intended, for advanced work is very rich indeed and should you wish to experiment with logic there's a set of logic gates, including various flip-flops.
As Windows programs go, this outshines many. Unusually, you can copy and paste a circuit into a worksheet on your word processor and, if you double click on it later, you can make it work or even change it. The print quality of the diagrams is good, but they do not stand much magnification.
Building circuits is fairly intuitive, with just a few irritations - you can't copy modules from one circuit to another and when you move a component its wires fall off and you have to draw them again. Still, you will get the hang of things quickly using some workcards and lots of ready-made circuits on the disc.
I'm just tiny bit miffed about the lack of curriculum materials - there's great potential here, at all levels of electronics work, but I just don't have the time to go find it. Never mind, it's still a good step on the way to building your own supercomputer.