That was my favorite activity when I was a preschooler and still remember sometimes the sheer panic I had when I took something too complicated apart and couldn't put it together. But most of the times I'd manage to figure out the puzzle back into one working thing. Worst problem I had was with screws of different sizes, sometimes I'd fit a square peg into a round hole.

I fondly remember how I electrocuted myself a few times too! One of my first stupid things I did which I still remember was to cut off a plugged clothing iron (with a metallic scissors). Maybe it's safer for kids nowadays but they're missing the real fun.