What do sweet potatoes wear to bed?

This joke is a pun that combines the word “yams,” which sweet potatoes are often confused with or colloquially referred to, and “jammies,” a colloquial term for pajamas. The humor comes from the playful use of language, taking the familiar concept of wearing pajamas to bed and applying it whimsically to sweet potatoes, which obviously do not wear clothing. By creating the portmanteau “yammies,” the joke playfully suggests that if sweet potatoes did wear bedtime attire, it would be something akin to pajamas but with a name that reflects their identity as yams (sweet potatoes), hence “yammies.”