Fun Question: (1 bonus point)
Four gentlemen came to sit-in in my lectures on Monday. Make a wild guess on the purpose of thier visit.
(Note: Fermi-style guesstimate will not help you here.)

The students' responses: (verbatim, unsorted)

1.  Observe you teaching a class you mention alot in emails to them.
2.  To either learn your teaching methods by observing or to evaluate the way
      you teach, I'm guessing.
3.  Evaluate your performance so as to determine whether you are suitable
      for promotion to be the Director of University Scholoars Programme
4.  Peer reviewing.
5.  To see whether there are hot girls in our class. (Wild!)

6.  They are free and thus, would like to sit in to know more. This is in
      view of the conference related to computer science occurring in NUS
      in this period of time.
7.  To see you teach for guidance and tips?
8.  To observe the lesson to study from you your good teaching method, some
      IT knowledge, and maybe for fun also (I guess you also attend to 
      other professors' lectures, right?)
9.  To see how the USP class is run and the way it is being taught.
10. They were here to study and evaluate your teaching style. Want to
      introduce the module/content to other department/universities.
      Exchange pointers.

11. Too see how the lectures are conducted.
12. They came to evaluate Prof Leong and the content taught in the
      UIT2201 class.
13. [Course evaluation] --- some of them might be curious what the 
      benchmark of USP modules is like. :-) (One of them was dozing off though.)
14. Since yhey were not carry any paper or notes, but came empty-handed, my
      guess is that you invited them on a casual basis (over lunch, perhaps?)
      to sit in and get a sense of either how you conduct your lectures or
      (more likely reason) to learn a bit about artificial intelligence, 
      since the topic itself sounds interesting and they would probably have
      been piqued by it.
15. To see how the lectures are being conducted and to assess whether this
      method of teaching fits the USP's model.

16. Learn how to give lectures from you.
17. I know that Dr Don does research with mental models and representations,
      so I'm guessing that he especially was interested in the AI topics.
18. To investigate(/ or appreciate?) the progress of IT education in Singapore!
      (...maybe in NUS only or maybe in USP only.)