Cannot figure out what is wrong

[sparkz_alot]

return is a value. in order to use it you need to assign it to a variable:

def ask_weight():
    lbs = float(input("How many pounds do you weigh? "))
    return lbs
weight = ask_weight()
print str(weight)

If you are using Python 2.x as was suggested, you should be using 'raw_input' rather than 'input'.