Not everything in Ruby is a full-fledged class; for instance, you can't extend Fixnum.
[I am apparently completely wrong here].
[I am apparently completely wrong here].
irb(main):001:0> class Fixnum
irb(main):002:1> def doit
irb(main):003:2> puts "hey"
irb(main):004:2> end
irb(main):005:1> end
=> nil
irb(main):006:0> 2.doit
hey
=> nil
irb(main):007:0>
class Fixnum
def +(other)
42
end
end