input: Write a python function to find the most common word in a given sentence.
  assert most_common_word('The cat in the hat sat on the mat') == 'the'
  assert most_common_word('Hello world Hello') == 'Hello'
  assert most_common_word('one fish two fish red fish blue fish') == 'fish'
output: def most_common_word(sentence):
      from collections import Counter
      words = sentence.split()
      word_counts = Counter(words)
      most_common = word_counts.most_common(1)
      return most_common[0][0]