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:

no

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. When Prolog searches for alternative ways of satisfying a goal, this is know as ‘backtracking’.

^[1] If nothing happens, 1) check your typing, 2) make sure you haven't forgotten the full-stop.