Small notes placed where they will be seen. They hold a thought outside a head so it need not be carried.
Middle English stikken (to fasten by piercing or adhering), from Proto-Germanic *stikan; the small adhesive note is named for staying put, which is the entire reason to write on one.
A small note placed where it will be seen, holding a thought outside a head so it need not be carried there. It works only because it stays stuck until someone reads it.