WTF?

console.rb

2.0.0-p594 :036 > x
 => nil 
2.0.0-p594 :037 > x = {}
 => {} 
2.0.0-p594 :038 > y = x
 => {} 
2.0.0-p594 :039 > z = x
 => {} 
2.0.0-p594 :040 > y["foo"] = "bar"
 => "bar" 
2.0.0-p594 :041 > z
 => {"foo"=>"bar"} 
2.0.0-p594 :042 > y
 => {"foo"=>"bar"} 
2.0.0-p594 :043 > x
 => {"foo"=>"bar"} 
2.0.0-p594 :044 > a = "asd"
 => "asd" 
2.0.0-p594 :045 > b = a
 => "asd" 
2.0.0-p594 :046 > c = a
 => "asd" 
2.0.0-p594 :047 > b = "xyz"
 => "xyz" 
2.0.0-p594 :048 > a
 => "asd" 
2.0.0-p594 :049 > b
 => "xyz" 
2.0.0-p594 :050 > c
 => "asd" 
2.0.0-p594 :051 > 

Why is a Hash different from a String?