input: Write a function to replace a given string with another string within a sentence. The function should take three parameters: the sentence, the old word, and the new word. It should return a new sentence with the old word replaced by the new word.
  
  def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)
  
  assert replace_word("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_word("The dog was running in the park", "running", "sitting")=="The dog was sitting in the park"
  assert replace_word("Python is an awesome language", "awesome", "great")=="Python is a great language"
  assert replace_word("The sun is shining brightly", "shining", "dark")=="The sun is dark brightly"
  assert replace_word("The quick brown fox jumps over the lazy dog", "fox", "cat")=="The quick brown cat jumps over the lazy dog"
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)