Mine

describe 'rand' do
  # fails:
  it "should be mockable (rand)" do
    Kernel.should_receive(:rand).and_return('foo')
    rand()
  end 

  # passes:
  it "should be mockable (Kernel.rand)" do
    Kernel.should_receive(:rand).and_return('foo')
    Kernel.rand()
  end 
end