What you design should be very very simple to someone when she starts to play with it.But as she plays more and more she should discover gradually(exponentiall

What you design should be very very simple to someone when she starts to play with it.But as she plays more and more she should discover gradually(exponentially) the great underlying complexity in order to achieve the simplicity on the surface. All the great codebases,machines,mathematical equations,laws of nature,even a bacterial cell seem to have this property.