[Bf-python] global name is not defined?

[naruto canada]

hi

what is the difference between blender python and regular python?
I try this example that works in regular python interpreter but got an
error with blender python,
...
def square(x): return (x * x)
def sum_of_squares(x, y): return (square(x) + square(y))
print sum_of_squares(3, 4)
...
NameError: global name 'square' is not defined

What's the difference between the two environments?
Thanks.