Immutable Object is Useful

From some talk videos I watched this style of code, Let's start with a simple example:

class Fish def initialize(age) @age = age end def say() "My age is #{@age}" end end fish = Fish.new(3) fish.say() #=> My age is 3 

This is called immutable or functional object. It's solid and cannot be broken after created.

Then how to change the age in sample?

class Fish #added def with_age(age) Fish.new(age) end end fish.with_age(5).say() #=>My age is 5 

This code style is fun and useful. It'll reduce the change of object.

What's your idea?