She's a witch, and talks about proving it to him by shape-changing into a cat or dog. He doesn't believe it, and is preparing to go swimming. She does whatever preparation is necessary for a shape-change, and apparently only has to state what she wants to change into, but at his suggestion she changes into
a swimsuit, literally.
I thought it might have been a Fredric Brown story, as he wrote several humorous stories involving shape-changing (e.g., "Abominable", "Bear Possibility", "Too Far"), but it doesn't appear in From These Ashes, so I'm probably wrong.