u n'ai pas faim, hein? -Si, j'ai faim-You're not hungry, right? -(Yes) I'm hungry
-Tu n'es jamais allé à Paris? -Si, j'y suis allé la semaine dernière-You've never been to Paris? -(Yes) I went there last week
-Tu ne comprends pas? -Si, je comprend -You don't understand? -(Yes) I understand
Si seems to be a very useful word. After all, in English, it's not always clear how you should give a positive answer to a negative question - do you answer with "No, what you said is wrong, I am going to do such-and-such" or do you answer with "Yes, I am going to do such-and-such"? Si gets around this ambiguity by contradicting the question and giving an affirmative answer, all in one two letter word.
All that said, I don't think I have once used si! It's just not very natural for an English speaker to use, so I've found it hard to remember to deploy it at an appropriate moment.
Definitely something to try and crow-bar into my next few conversations in French.