I packed my suitcase for a trip to the desert.

The humor in this desert travel joke springs from a delightful mix of absurdity and a dash of optimistic delusion. Our traveler, heading to a place famously devoid of water, packs an item specifically designed for it: a swimsuit. This immediately sets up an amusing contrast (Review), playing on the common knowledge that deserts and swimming don't exactly go hand-in-hand. The idea of preparing for a mirage, a known optical illusion, as if it were a real swimming opportunity, is where the initial chuckle begins.

The punchline then cleverly brings in a bit of wordplay and a dose of reality. Mirages, those shimmering illusions of water often seen in hot, flat environments, are a classic desert phenomenon, born from light bending through air of varying temperatures. The wife's comment, "you were just thirsty," grounds the fantastical idea of swimming in an illusion with the very real, and very common, experience of being dehydrated in the desert. It's a neat twist, using "thirsty" to mean both literally needing a drink and perhaps a bit too eager for an imagined oasis.