John casually references some urban myth that Sherlock's never heard of, and he interrogates him out of curiosity. It turns out Sherlock's never heard of a lot of urban myths, nor seen MythBusters. And when he watches the show, he notices (as many have) that the science is crap. So he sets out to test for himself the myths that interest him. Hilarity ensues. May or may not include YouTube documentation of these exploits.