Prankster Jack Vale has come up with a rather neat - and funny - trick.
He hovers around people while pretending to be on the phone - and then, in this pretend phone call, starts talking about the strangers he's hovering around. It's called the 'Making People Paranoid' prank - and it works.
Vale tried this prank at a conference back in February with hilarious results.
But when he tried it recently on two American police officers? Things went slightly differently.
"All I did was describe these cops to someone on the phone," Vale says on YouTube, "and this guy went crazy!"
He most certainly did...