What does it mean to put your finger in the cup?

I often see an old saying that when staying in a hotel, I put two cups on the door handle, saying it is for safety. I don't know if it's safe. Now all doors have anti-theft chains.

Fingers are affected by buoyancy, and buoyancy is bound to react.

Look at the relationship between this girl and you. If she is just an ordinary friend, drawing a circle is a sign of her boredom. If it is a boyfriend and girlfriend, it may be a hint.