This one has been working very nicely for me. I recently read something about how girls always open their mouths when they put on mascara and noticed it's so true.

Me: Hey I have a question, why do girls always open their mouths when they put on mascara
Girls: blah blah blah (varies greatly)
Me: You know I've been wondering about this for awhile now and scientists have been doing research for years and have finally found the answer. You wanna know why girls open their mouths when they put on mascara?
Girls: Why?
Me: Because their stupid, that's why haha

I usually do this after opening/have nothing to say/filler and while playing with my phone or their phone. Works wonders on getting them to talk while I'm nonchalantly doing something. And the ending ALWAYS never fails to make them giggle. Also, you don't need to do the whole scientist thing plus the "because their stupid". The question in itself makes for good conversation and opens opportunities to bust on them and whatever. Could also be used as an opener.
Cheers.

I like the idea, well done.