ITP2 - OOP Tutorials - Inheritance - Exercises

Given the following code...

bob = Person.new
bob.hi

And the corresponding error message...

NoMethodError: private method `hi' called for #<Person:0x007ff61dbb79f0>
from (irb):8
from /usr/local/rvm/rubies/ruby-2.0.0-rc2/bin/irb:16:in `<main>'

What is the problem and how would you go about fixing it?