If someone can tie a cherry stem into a knot with their tongue, that suggests they may be a good kisser due to the oral dexterity required to perform the feat. It is a common trick performed at bars due to the presence of Maraschino cherries as drink garnishes.
If you cannot master the technique of tying a cherry stem into a knot, faking it is always an option. Simply tie a cherry stem into a knot beforehand and place it into your mouth when no one's looking. Place an untied stem in your mouth and make the appropriate motions for a few moments, then pull out the prepared stem to impress your friends.