I did not know that.

gistfile1.txt

[1] pry(main)> Class.new.respond_to?(:define_method)
=> false
[2] pry(main)> Module.new.respond_to?(:define_method)
=> false
[3] pry(main)> Class.new.send(:define_method, :yup) {}
=> #<Proc:0x00000100a08cb8@(pry):3 (lambda)>
[4] pry(main)> Module.new.send(:define_method, :yup) {}
=> #<Proc:0x00000100d8f468@(pry):4 (lambda)>

As @JEG2 kindly pointed out: https://twitter.com/JEG2/status/370268287675805696

facepalm

For the record, Module#respond_to? takes an optional parameter to include private methods:

[9] pry(main)> Module.new.respond_to?(:define_method, true)
=> true