Do tell.

First example vid I found using those keywords: https://m.youtube.com/watch?v=kXaOP8O4HpI

Written description: https://navyaviation.tpub.com/14329/Figure-6-52-One-Method-O...

...although I haven't found any explanation online why it works. This method has probably been known for over a century, so I suspect it may be in some old physics books.

Which seems to show that you use the pencil graphite as an electrode and spark it to ground. The 'fatness' of the spark determines the polarity.