irb(main):001:0> Foo = 'a'
=> "a"
irb(main):002:0> p Foo
"a"
=> nil
irb(main):003:0> p defined? Foo
"constant"
=> nil
irb(main):004:0> Foo << "b"
=> "ab"
irb(main):005:0> Foo.freeze
=> "ab"
irb(main):006:0> Foo << "c"
TypeError: can't modify frozen string
        from (irb):6:in `<<'
        from (irb):6