they go to canada to buy beer, fez loses his green card and the border cops are questioning them. tommy chong's questioning goes like this:

canadian- what are you doing in canada?
chong- what are you doing in canada?
canadian- what's your business in canada?
chong- what's your business in canada?
canadian- leave canada please
chong- you leave canada please!
canadian- dirty hippie.

ok, so it's funnier when you actually see it.