input: Write a function that takes in two strings and returns an integer indicating the number of common substrings between the two strings. The function should iterate through all possible substrings of the first string and check if each substring exists in the second string. If it does, a counter should be incremented. Finally, the function should return the counter value.
  
  def num_common_substrings(s1, s2):
      count = 0
      for i in range(len(s1)):
          for j in range(len(s2)):
              if s1[i] == s2[j]:
                  count += 1
      return count
  
  assert num_common_substrings("abcd", "bae")==2
  assert num_common_substrings("hello", "world")==2
  assert num_common_substrings("abc", "def")==0
  assert num_common_substrings("same", "same")==4
  assert num_common_substrings("", "")==0
  assert num_common_substrings("longlongstring", "short")==3
output: def num_common_substrings(s1, s2):
      count = 0
      for i in range(len(s1)):
          for j in range(i + 1, len(s1) + 1):
              if s1[i:j] in s2:
                  count += 1
      return count