diff --git a/text_to_sql_query.rb b/text_to_sql_query.rb
index 8c96f39..bd40ba5 100644
--- a/text_to_sql_query.rb
+++ b/text_to_sql_query.rb
@@ -21,33 +21,33 @@ class TextToSqlQuery
   private
 
   def generate_sql(tree)
-    @first_key = tree.keys.first
-    case @first_key
+    first_key = tree.keys.first
+    case first_key
     when :DEFAULT_COLUMN
-      ["#{@default_field.to_s} LIKE ?", "%#{tree[@first_key]}%"]
+      ["#{@default_field.to_s} LIKE ?", "%#{tree[first_key]}%"]
     when :OPERATOR_OR
-      generate_expression_for_logical_operator(:OR, tree[@first_key])
+      generate_expression_for_logical_operator(:OR, tree[first_key])
     when :OPERATOR_AND
-      generate_expression_for_logical_operator(:AND, tree[@first_key])
+      generate_expression_for_logical_operator(:AND, tree[first_key])
     when :OPERATOR_NOT
-      @not_array = generate_sql tree[@first_key]
+      not_array = generate_sql tree[first_key]
 
-      if @not_array.length < 2
+      if not_array.length < 2
         raise "There should be more than 1 element for expression following NOT operator"
       end
 
-      @not_expression = @not_array.first
-      @not_params = @not_array[1..]
+      not_expression = not_array.first
+      not_params = not_array[1..]
 
-      ["NOT #{@not_expression}"] + @not_params
+      ["NOT #{not_expression}"] + not_params
 
     else
       # key is column name
-      @mapping = @fields_mappings[@first_key.to_sym]
-      if @mapping.nil?
-        ["#{@default_field.to_s} LIKE ?", "%#{tree[@first_key]}%"]
+      mapping = @fields_mappings[first_key.to_sym]
+      if mapping.nil?
+        ["#{@default_field.to_s} LIKE ?", "%#{tree[first_key]}%"]
       else
-        ["#{@mapping.to_s} LIKE ?", "%#{tree[@first_key]}%"]
+        ["#{mapping.to_s} LIKE ?", "%#{tree[first_key]}%"]
       end
     end
   end
@@ -57,23 +57,23 @@ class TextToSqlQuery
       raise "There should be two array elements for #{operator.to_s} operator"
     end
 
-    @first_operand = generate_sql operator_array.first
-    @second_operand = generate_sql operator_array.last
+    first_operand = generate_sql operator_array.first
+    second_operand = generate_sql operator_array.last
 
-    if @first_operand.length < 2
+    if first_operand.length < 2
       raise 'There should be more than 1 element in first operand array'
     end
 
-    if @second_operand.length < 2
+    if second_operand.length < 2
       raise 'There should be more than 1 element in second operand array'
     end
 
-    @first_operand_expression = @first_operand.first
-    @first_operand_params = @first_operand[1..]
+    first_operand_expression = first_operand.first
+    first_operand_params = first_operand[1..]
 
-    @second_operand_expression = @second_operand.first
-    @second_operand_params = @second_operand[1..]
+    second_operand_expression = second_operand.first
+    second_operand_params = second_operand[1..]
 
-    ["#{@first_operand_expression} #{operator.to_s} #{@second_operand_expression}"] + @first_operand_params + @second_operand_params
+    ["#{first_operand_expression} #{operator.to_s} #{second_operand_expression}"] + first_operand_params + second_operand_params
   end
 end
\ No newline at end of file