Ever watch your horse out in the field carrying their head so beautifully, completely on their own? Then you just wonder, "How come they don't do that when I ride them?"
The smile is a simple trick that is great to use when you are first starting clicker training. It helps establish the association of the click with a reward, and it helps build the horse's (and your!) confidence with clicker training. Initially, reward any attempt to lift the lip, even if it is to grab at the treat in your hand. Then, once they are consistently lifting their nose, you can focus on keeping them out of your space and not being grabby. Of course, if your horse is being aggressive, you will want to correct from the beginning so you don't teach bad habits.
This is a game we chose to practice because it incorporates a couple of different cues! Because of this, it would probably be easier if your horse learns a couple of things before playing:
As a girl, I always dreamed of having a horse that I could not only ride, but who would also run up to me, want to spend time with me, play, and be generally well behaved and happy. It was the type of thing that I would daydream about constantly and yearn for the day I could have my own horse.