Mug may be sipped and gulped of

* Mug#sip takes a drink consuming 1 percent of the Mug's contents
* Mug#gulp takes a drink consuming 10 percent of the Mug's contents
* s/percent_empty/percent_filled/ and fixes semantic logic

mug.rb

@@ -1,21 +1,36 @@
 class Mug
-  attr_reader :color, :content, :percent_empty
+  attr_reader :color, :content, :percent_filled
 
   def initialize(color=:white)
-    @color         = color
-    @percent_empty = 100
+    @color          = color
+    @percent_filled = 0
   end
 
   def fill(liquid=:coffee)
-    @content       = liquid
-    @percent_empty = 0
+    @content        = liquid
+    @percent_filled = 100
   end
 
   def filled?
-    percent_empty < 100
+    percent_filled > 0
   end
 
   def empty?
     !filled?
   end
+
+  def sip
+    drink(1)
+  end
+
+  def gulp
+    drink(10)
+  end
+
+  private
+
+  def drink(percent)
+    raise "You're all out of #{content}" if empty?
+    @percent_filled -= percent
+  end
 end