input: Write a function to find the most common words in a given string.
  assert most_common_words("This is a sample string.") == [('This', 1), ('is', 1), ('a', 1), ('sample', 1), ('string', 1)]
  assert most_common_words("hello hello world world world") == [('world', 3), ('hello', 2)]
  assert most_common_words("test test test test") == [('test', 4)]
output: def most_common_words(text):
      from collections import Counter
      words = text.split()
      count = Counter(words)
      return count.most_common()