Let's say, that we have a method called calculate_speed.
We want this method to be present in classes
Car, Rocket, and Ship.
One way to do this is by adding the method
in each of these classes:
But there is a problem with this approach.
When we want to change
puts "current speed is now visible on your dashboard"
to
puts "current speed is now visible on your homepage"
we'll need to go into all
the classes and change
each method individually.
This isn't ideal.
We want this method to be at only one place,
so that if we
make changes to it,
we get to use the updated method everywhere else.
We can do that by putting this method in a module.
Now, any class that needs to
calculate speed can include
the module and can do the speed calculation.
Try changing the message in
calculate_speed and you'll
see that the result gets updated
in all the instances.