Consider the following:

module Car
  def drive
    puts "brrrrr"
  end
end

module Train
  def drive
    puts "cho-choo!"
  end
end

class TeaCup
  include Car
end

tea = TeaCup.new

We have a Car and Train module, but we only include the Car in the TeaCup.

Accordingly I would expect a TeaCup to still be a TeaCup - but if we adopt the behaviour of a Car, should a TeaCup also be a Car?

Test yourself: what would you expect?

puts "tea.is_a? TeaCup: #{tea.is_a? TeaCup}"
puts "tea.is_a? Train: #{tea.is_a? Train}"
puts "tea.is_a? Car: #{tea.is_a? Car}"

The answers would surprise you:

# => tea.is_a? TeaCup: true
# => tea.is_a? Car: true
# => tea.is_a? Train: false

But why and how?

Straight from the documentation:

Returns true if class is the class of obj, or if class is one of the superclasses of obj or modules included in obj.

Ancestor Chains

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