Same with Kinder Surprise. When I was a young child, I got a flywheel mouse with multiple interlocking precision gears. I must have spent hours disassembling and reassembling it, instead and studying how all the pieces fit together. When I was an older child I got a single-piece plastic gorilla with green fur. It turned over in my hands a few times and probably threw it away.

Also, those Bionicle Happy Meal toys got me hooked on Lego Technic. Most addictive gateway drug ever.