Why does this not work?

ruby-1.9.2-p0 > b = binding
 => #<Binding:0x9adc81c>
ruby-1.9.2-p0 > eval("x = 5", b)
 => 5
ruby-1.9.2-p0 > x
NameError: undefined local variable or method `x' for main:Object
	from (irb):6
	from /home/martin/.rvm/rubies/ruby-1.9.2-p0/bin/irb:17:in `<main>'
ruby-1.9.2-p0 > eval("x")
NameError: undefined local variable or method `x' for main:Object
	from (irb):7:in `eval'
	from (irb):7:in `eval'
	from (irb):7
	from /home/martin/.rvm/rubies/ruby-1.9.2-p0/bin/irb:17:in `<main>'
ruby-1.9.2-p0 > eval("x", b)
 => 5

martin