input: Write a python function to find all possible substrings of a given string.
  assert find_substrings("abc") == ['a', 'ab', 'abc', 'b', 'bc', 'c']
  assert find_substrings("ab") == ['a', 'ab', 'b']
  assert find_substrings("a") == ['a']
output: def find_substrings(s):
      substrings = []
      for i in range(len(s)):
          for j in range(i + 1, len(s) + 1):
              substrings.append(s[i:j])
      return substrings