Hawking's Mum

Alright, listen up, buttercup. So, this Hawking dude just says "My mum." That's it. It's supposed to be funny because... well, it isn't. People were expecting him to say something smart or science-y, but instead, he just mentions his mom. It's like when you try to bake a cake and end up with a pile of goo – unexpected and kinda sad. Also, since you are reading this, you probably need help understanding jokes.