diff --git a/railties/guides/source/active_support_core_extensions.textile b/railties/guides/source/active_support_core_extensions.textile
index d601e9ea295859d6265e5ba8f4270dfd2bc766a7..c1046a3b63c90b0363319b8ba94ba0fa39747d1d 100644
--- a/railties/guides/source/active_support_core_extensions.textile
+++ b/railties/guides/source/active_support_core_extensions.textile
@@ -440,14 +440,16 @@ NOTE: Defined in +active_support/core_ext/kernel/reporting.rb+.
 
 h4. +in?+
 
-The predicate +in?+ tests if an object is included in another object. An +ArgumentError+ exception will be raised if the argument passed does not respond to +include?+.
+The predicate +in?+ tests if an object is included in another object or a list of objects. An +ArgumentError+ exception will be raised if a single argument is passed and it does not respond to +include?+.
 
 Examples of +in?+:
 
 <ruby>
+1.in?(1,2)          # => true
 1.in?([1,2])        # => true
 "lo".in?("hello")   # => true
 25.in?(30..50)      # => false
+1.in?(1)            # => ArgumentError
 </ruby>
 
 NOTE: Defined in +active_support/core_ext/object/inclusion.rb+.