Toy helicopter:

Physical principle: wind up, pretend play.

age group: 3-5 years old

interaction:  child uses imagination, winds up toy, pretends it is actual helicopter.

construction/safety: thin sheet metal can be stamped into shape cheaply.  making the helicopter blades will be more expensive, and the wind-up mechanism will have to be geared to not spin the blades too fast, which would be another safety hazard.

what I learned:  something simple often turns out to be more complicated than it seems.