Ruby Instance Array Is Modified in a Separate Object

Lets say I create an object with an array. I then create another object that takes said array and modifies it within its own scope. If I try to access original array from the first object again, shouldn't I have the unmodified version of the array?

Example code:

class Test
    attr_accessor :g

    def initialize
        @g = [1,2,3]
    end

    def do_stuff
        Test_Two.new(@g).modify
    end

end

class Test_Two
    def initialize(h)
        @h = h
    end

    def modify
        @h[0] +=1
    end

end


t = Test.new
puts "#{t.g}"
t.do_stuff #this shouldn't modify t.g
puts "#{t.g}"

Expected output:

[1,2,3]
[1,2,3]

Actual output:

[1,2,3]
[2,2,3]

Weirdly enough, if I make g an integer, I get what I expect:

class Test
    attr_accessor :g

    def initialize
        @g = 1
    end

    def do_stuff
        Test_Two.new(@g).modify
    end

end

class Test_Two
    def initialize(h)
        @h = h
    end

    def modify
        @h +=1
    end

end


t = Test.new
puts "#{t.g}"
t.do_stuff #this shouldn't modify t.g
puts "#{t.g}"

Output:

1
1