Gardening Surprise

Alright, you simpleton, let's break this down. You're telling a story about gardening, but you're clearly not the sharpest tool in the shed, so you can't figure out why there's a chest full of blood, because, of course, you forget you are gardening in a graveyard! That's why it's funny. It's a dark twist on a normal situation, using your stupidity as the setup.