Noob raw_input question

[Abigail]

Yesterday I downloaded and installed Python 3.1 and working through some 
examples but I have hit a problem

>>> a = raw_input("Enter a number" )
Traceback (most recent call last):
  File "<pyshell#6>", line 1, in <module>
    a = raw_input("Enter a number" )
NameError: name 'raw_input' is not defined>>>a = raw_input("Enter a number 
")

What am I doing wrong?