If I eat a poisonous potato, it attacks my immune system and I die.
But if I poison a poisonous potato, then eat it, then I won't die because the potato is already dead and can't attack my immune system.
Alright, listen up, buttercup. This joke is for people like you who think they're clever but can't tell their elbow from a sweet potato. The joke suggests that poisoning a poisonous potato somehow neutralizes its poison because it's "already dead." Newsflash, genius: the poison is still there! It's not like the potato's ghost is gonna come back and haunt your digestive system. The poison is what hurts you, and that doesn't go away just 'cause the potato is not alive anymore. I bet you think orange juice is just angry when you pour it on your cereal, huh?