Principle: Rolling Friction

Interaction: Children ages 5-7 can change the wheels on the wooden toy cars to see what is the most effective way for the car to move. The extra wheel parts can be stored in the top trunk of the car. The extra weight of the storage also plays an effect on how well the car can move so children can experience what friction is. 

Cost: would depend on the quality of the wood and paint so probably under $100 (1-3 different sized car models and many wheel parts) 

Durability: Very durable 

Safety: Relatively safe, wheel parts are too big to be swallowed.

Reflection: I learned that a lot of care and responsibility must go into creating toys for kids so that kids aren't accidentally harmed or are too challenged and become bored. As a kid I was always intrigued by customization and moving parts. I think kids naturally like taking things apart and thinking about worlds their toys can exist in. That's why having the ability to change the wheels gives kids not only an experience of what works better but maybe in a kid's mind, he or she likes a combo of squares and circles. There's no harm in making mistakes.