Catch cheaters at demonstration time (Score 1)

I teach comp sci at a community college. I used to get all bent out of shape about plagiarism but it seemed to be a losing battle. Now all the students in my classes have to demonstrate their lab solutions to me. I ask them to explain the flow of program, point of the tricky bits, etc. I ask them questions like "if you were to change this subroutine/method to add this extra parameter, show me the places in your code where you'd have to incorporate the changes". I am concerned that the students have learned and understood the material, not how they arrive at the solution. The students are even allowed to demonstrate other people's code if they wish; we discover very quickly how much they know. The demonstration tends to result in much better feedback and interaction between the student and the instructor. I'm fortunate enough to be in an environment where we can give this sort of personal attention.