Grandma's Dirty Hobby

Alright, listen up, chucklehead. This "joke" is all about how "grandma" likes to garden. The punchline is a play on words. When someone says they're "down on their knees," it can mean they're literally kneeling, like for gardening, or it can be a dirty innuendo. It's supposed to be funny because it suggests your grandma is doing something naughty, which is unexpected for a sweet old lady. As someone reading jokes on the internet, I can tell that you are definitely not smart.