Page 94 - Berndt Sundsten "My First Book of Knots: A Beginner's Picture Guide"

Fisherman’s loop (true lover’s knot)

Here’s a knot you can use if you want to see if someone is interested in you.
Make a fisherman’s loop and give it to the person you’re in love with. If you
get it back tightened, everything is clear without either of you having to say
anything. It’s said that sailors used to send a thin string or silk ribbon with
this knot to their girlfriends to see if they were still in love with and loyal to
them. If the knot came back properly set, everything was as it should be. If
not, then things were over.