A difficult-to-get joke.

Today our pbl facilitator decided it was appropriate to share with us a..... rather.... actually it's indescribable... so she just.... shared with us a joke. I couldn't really get it at first, so let's see if anyone could. It was at the end of our session when all of us had already packed up our stuff and was just about to leave when:

Faci : Ei you guys wait lah !!! Before you all go ah, i want to ask you all a question...
PBL members : (All very excited!!)
Faci : How do you all determine arr whether somebody is a guy or girl ???
PBL members : ..........
Faci : Come on you guys, make a guess. How do you all know ??
PBL members : ...... (Looks at each other..) .......
Faci : Haiyah like that also you all don't know arr... You just take down the genes...
PBL members : ???
Faci : Aiyoh come on... (Getting very excited) then how do you all tell whether somebody is a guy or a girl ?!?!
Me : ....urm.... karyotyping...?
Faci : (Getting more excited) Haiyoh you all don't geeeet iiiiit aaaahh ?!?!?!
Me : (Whispering to nearest member while glancing warily at excited faci...) ... yes but... then what do we to the gene ??????
Faci : (Getting very frustrated) Haiyah you all very slow lahhh...
PBL members : (Mumble mumble + look at each other somemore...)
Faci : (Nearly out of breath...) You all rrreeeeaaalllyyy dddooonnn'tttt ggeeeetttt iittttt aaaarrrr ??? To see whether somebody is a guy or a girl... You just take down their JEANS !!!