For me, it was replicas.
The first replica I ever saw was in an Auto Trader magazine when I was 9 years old. It was a Toyota MR2-based Ferrari F355 replica, not unlike the one pictured above. At the time, I didn't understand the concept of replicas at all. I didn't understand the reason for their existence.
I remember I kept asking my father how it was possible - how could a Toyota MR2 look like a Ferrari? A Toyota is Japanese, a Ferrari is Italian, how was that possible? Dad, it's on sale for such a cheap price, does the word "replica" have something to do with it? What is a replica Dad?
It bugged me for a week until I finally understood. Now whenever I see a replica, I'm forced to remember that slightly embarrassing childhood memory.
What about you, fellow Opponauts? Have you ever struggled to understand anything vehicular related as a child? Electric windows? Windscreen washers? Brakes? Do tell!