print "Start entering words:"
previous = []
word = raw_input()
while word not in previous:
    previous.append(word)
    word = raw_input()
print "You already entered " + word + "."
print "You listed", len(previous), "distinct words."

(6 points)

def acronym(phrase):
    a = ''
    for word in phrase.split():
        a += word[0]
    return a.upper()

Or, for those who like to use list comprehensions,

def acronym(phrase):
    return ''.join([word[0] for word in phrase.split()]).upper()

def sliding(word, num=3):
  for i in range(len(word)+1-num):
    print ' '*i + word[i:i+num]

def pairSum(data, goal):
  for a in range(len(data)-1):
    for b in range(a+1, len(data)):
      if data[a] + data[b] == goal:
        return True
  return False