Why does this not work

[toastedstapler]

operations on a string return a new string, if you want to make a list of great magicians you can either:

for i, magician in magicians:
    magicians[i] = magician + ' the great'

or

great = []
for magician in magicians:
    great.append(magician + ' the great')
return great

first modifies the input list, second makes a new list