If you have to explain a joke, that usually means it's not a very good joke. But, if you're telling a math joke, maybe it just means your audience never enjoyed algebra as much as you did. Either way, these videos help explain to the less able mathematicians out there why they're funny.
No really, they are funny. Promise. Maybe. Fortunately, the explanations—delivered dead-pan by stand-up comedian-come-mathematician Matt Parker—are equally amusing. Enjoy, both of 'em. [Numberphile]