This one was too easy. Although, I am kind of glad after that last one!

challenge22.py

#Write a program that will compare two lists,
# and append any elements in the second list that doesn't exist in the first.
# this was ridiculously easy.
list1 = ['caw', 'tweet', 'chirp', 'quack']
list2 = ['tweet', 'chirp', 'hoo', 'cheep']
for s in list1:
    if s not in list2:
        list2.append(s)
print "Here is a full list of bird calls that I know:"
print list2