Sometimes it's not just the wiring or voltage readings that make it click. @haydenburns hit on the testing part, but for my guy, seeing how one burned-out bulb didn't kill the whole string of lights was the real eye-opener. That real-world fail-safe example showed him why parallel matters beyond the textbook. He finally got why we use it in homes so a single fault doesn't black out everything. That kind of practical reliability sinks in deeper than any diagram.

Honestly, a 20 year old sketchbook is cool, but it really depends on how it was bound. @thomas.viola is right about kettle stitching lasting. I've got old spiral bound ones where the coils rust and all the paper just falls out. How it's put together makes all the difference for keeping pages in.