Wait. What?

class_variables.rb

1.9.3p286 :151 > module P
1.9.3p286 :152?>   @@b = 1
1.9.3p286 :153?>   end
 => 1 
1.9.3p286 :154 > class A
1.9.3p286 :155?>   include P
1.9.3p286 :156?>   end
 => A 
1.9.3p286 :157 > class B
1.9.3p286 :158?>   include P
1.9.3p286 :159?>   end
 => B 
1.9.3p286 :160 > class B
1.9.3p286 :161?>   @@b += @@b
1.9.3p286 :162?>   end
 => 2 
1.9.3p286 :166 > module P
1.9.3p286 :167?>   @@b
1.9.3p286 :168?>   end
 => 2 
1.9.3p286 :169 > module Q
1.9.3p286 :170?>   @@b = 4
1.9.3p286 :171?>   end
 => 4 
1.9.3p286 :172 > class B
1.9.3p286 :173?>   include Q
1.9.3p286 :174?>   end
 => B 
1.9.3p286 :175 > module P
1.9.3p286 :176?>   @@b
1.9.3p286 :177?>   end
 => 2 
1.9.3p286 :178 > class A
1.9.3p286 :179?>   @@b
1.9.3p286 :180?>   end
 => 2 
1.9.3p286 :181 > class B
1.9.3p286 :182?>   @@b
1.9.3p286 :183?>   end
 => 2 
1.9.3p286 :184 > module Q
1.9.3p286 :185?>   @@b
1.9.3p286 :186?>   end
 => 4 
1.9.3p286 :187 > 
1.9.3p286 :187 > class C
1.9.3p286 :188?>   include Q
1.9.3p286 :189?>   include P
1.9.3p286 :190?>   end
 => C 
1.9.3p286 :191 > class C
1.9.3p286 :192?>   @@b
1.9.3p286 :193?>   end
 => 4 
1.9.3p286 :194 >