merge vs merge!

merge.rb

main_hash = {}
K = 10000
start_time = Time.now
(1..K).each do |number|
  main_hash = main_hash.merge({number => number})
end
puts "with 'merge'"
puts "Time elapsed #{Time.now - start_time} seconds"
puts "Main hash key count: #{main_hash.keys.count}"

main_hash2 = {}
start_time = Time.now
(1..K).each do |number|
  main_hash2.merge!({number => number})
end
puts "with 'merge!'"
puts "Time elapsed #{Time.now - start_time} seconds"
puts "Main hash key count: #{main_hash2.keys.count}"