Answer:
def sillystring(input):
 output = ""
 vowels = ['a', 'e', 'i', 'o', 'u']
 for letter in input:
 if (letter.lower() in vowels):
 output += "oob"
 else:
 output += letter
 return output
Testing:
>>> print(sillystring('Hello, world!'))
Hooblloob, woobrld!