input: Write a python function to find the most common word in a given text.
  assert most_common_words("this is a test this is only a test") == "this"
  assert most_common_words("apple banana apple orange banana apple") == "apple"
  assert most_common_words("one two two three three three") == "three"
output: def most_common_words(text):
      from collections import Counter
      words = text.split()
      counter = Counter(words)
      most_common_word, _ = counter.most_common(1)[0]
      return most_common_word