h = {}
h["Greensleeves"] = "all my joy"
h

h["Greensleeves"] = "my delight"
h

key = "Modify me if you can"
h = { key => 1 }
key.upcase!

h[key]

h["Modify me if you can"]

h.keys

h.keys[0].upcase!                   

TypeError: can't modify frozen string
...

TypeError: can't modify frozen string
	from (irb):12:in `upcase!'
	from (irb):12

squares = [[1,1], [2,4], [3,9]]
results = {}
squares.each { |k,v| results[k] = v }
results

squares.inject({}) { |h, kv| h[kv[0]] = kv[1]; h }

class Array
  def into_hash(h)
    unless size % 2 == 0
      raise StandardError, "Expected array with even number of elements"  
    end
    0.step(size-1, 2) { |x| h[self[x]] = self[x+1] }
    h
  end
end
squares = [1,1,2,3,4,9]
results = {}
squares.into_hash(results)

[1,1,2].into_hash(results)

StandardError: Expected array with even number of elements
...

StandardError: Expected array with even number of elements
	from (irb):21:in `into_hash'
	from (irb):30

squares = { 1 => 1, 2 => 4, 3 => 9}
cubes = { 3 => 27, 4 => 256, 5 => 3125}
squares.merge!(cubes)
squares

cubes

squares = { 1 => 1, 2 => 4, 3 => 9}
cubes = { 1 => 1, 2 => 8, 3 => 27}
squares.replace(cubes)
squares

cubes[4] = 64
squares

defaults = {:verbose => true, :help_level => :beginner }
args = {}
requests.each do |request|
  args.replace(defaults)
  request.process(args) #The process method might modify the args Hash.  
end  

...

NameError: undefined local variable or method `requests' for main:Object
	from (irb):44