Box is a data structure that allows us to move our data from the stack to the heap.
Box is a struct known as a smart pointer that holds the pointer to our data on the heap.
Because Box is a struct with a known size (because it just holds a pointer), it is
often used as a way to store a reference to something in a struct that must know the size
of its fields.
Box is so common it can be used from anywhere:
Box::new(Foo { ... })