input: Write a python function to find the most common word in a given string.
  assert most_common_word("the quick brown fox jumps over the lazy dog the the") == "the"
  assert most_common_word("apple orange banana apple banana apple") == "apple"
  assert most_common_word("a b c a b a") == "a"
output: def most_common_word(s):
      words = s.split()
      word_count = {}
      
      for word in words:
          if word in word_count:
              word_count[word] += 1
          else:
              word_count[word] = 1
      
      most_common = max(word_count, key=word_count.get)
      return most_common