Mom writes on stationery while Dad rides the stationary bike.
Stationary [THINK: Adjective (A)]
Stationery [THINK: Envelope (E)]
“Stationary with an a…means “fixed in one place and not moving,” like a stationary bicycle at the gym.
Stationery with an e stems from the term stationer, which refers to “a person who sells the materials used in writing, such as paper, pens, pencils, and ink.””
[read more about it on blog.dictionary.com]