Lingerie
Explanation
The Joke
A woman presents what she calls "the ideal lingerie" -- but instead of something conventionally sexy, she has constructed an elaborate outfit made from burning magnesium, an opaque fog of hydrogen, and knitted fibers from rare elements. She describes it in purely technical, scientific terms and then commands "activate!" The outfit produces a dramatic cloud of fog and sparks around her body.
Her partner's reaction is the punchline: he says "I could just be naked" and she retorts "but where is the mystery?" The joke inverts the entire concept of lingerie. Lingerie is supposed to tantalize by partially concealing and partially revealing the body. This woman has taken the "concealment" aspect to an absurd scientific extreme -- engineering an outfit designed to maximally obscure her body with fog, fire, and rare materials -- while completely missing the point that the appeal of lingerie is in what it suggests, not in how elaborately it hides things.
The Humor
The humor comes from applying an engineer's literal, problem-solving mindset to romance and seduction. If lingerie works by creating "mystery," then logically the most mysterious lingerie would be one that makes it physically impossible to see anything at all. It is a classic SMBC move: taking a social convention, interpreting it with ruthless literalism, and arriving at an absurd but technically defensible conclusion. The man's exasperated suggestion that nudity would be simpler highlights how far the woman has overcomplicated something fundamentally simple.