To get Prolog to do some computation, you need to issue a goal. Type
|?- grandmother(liz, Who).
Type this exactly, including the capital W and the final full-stop, and, hit <return>. Prolog will respond:1
Who = harry
Next, type a semi-colon followed by <return>. Prolog will respond
Who = wills
Again, type a semi-colon followed by <return>. Prolog will respond:
This means that you have now received all the responses to the goal that Prolog can compute on the basis of the information supplied in your program file.