Why does this print something?

gistfile1.txt

require 'minitest/autorun'

module Kernel
  def sleep(n)
    puts "calling sleep"
  end
end

describe "stub" do
  it "stubs stuff" do
    Kernel.stub(:sleep, nil) do
      sleep(5)
    end
  end
end

gistfile2.txt

$ ruby bug.rb
Run options: --seed 13072

# Running tests:

calling sleep
.

Finished tests in 0.001058s, 945.1796 tests/s, 0.0000 assertions/s.

1 tests, 0 assertions, 0 failures, 0 errors, 0 skips

steve at thoth-2 in ~/tmp
$ ruby -v
ruby 2.0.0p0 (2013-02-24 revision 39474) [x86_64-darwin12.2.0]

steve at thoth-2 in ~/tmp
$ gem list minitest

*** LOCAL GEMS ***

minitest (4.7.0, 4.6.2, 4.6.1, 4.6.0, 4.3.3, 4.3.2)
minitest-rails (0.3)