Smiling on cue is a fun and cute party trick to teach a horse. The lip moving motion often dubbed as a smile is actually called "flehmen" which the horse uses to identify a strange smell. Some horses naturally do this more than others, most frequently stallions. However, horses of all ages and genders can learn this behavior as a trick. There are a couple ways to go about teaching your horse to smile, as outlined below.
Method # 1: Capturing the Behavior
For this method, you need some treats for your horse, and something strange smelling. You may have to try a couple different scents to find something that will get your horse's attention. Open the scent container a few feet away from your horse, and be ready with treats. If your horse lifts their lip at all in response to the new smell, reward with a treat. Offer a treat every time your horse repeats their lip lifting behavior. If it doesn't work, you can try a different scent or try the second method below.
Method # 2: Teaching by Touch
This method involves prompting the horse to lift their lip by touch. For this you will need the treats again, and perhaps a feather if needed. Try touching the horse's nose, lips or gums with your finger and be ready to reward quickly with a treat if they lift their lip up. You can also try the feather to see if that works better for your horse.
Whichever method you try, be sure to reward quickly as soon as the horse does the trick. You will likely need to reward for just a quick "smile" or even just moving their lip slightly at first, and gradually build up to the final trick. Practice until the horse is consistently offering something with their lip, and continue to improve and build on it. The video below shows a good example of how the smile training process starts with a new horse.
Adding a Cue
Once the horse is repeatedly offering to raise their lip, you can start adding a cue every time the horse smiles, right before you give them the treat. A cue can be verbal, like "smile," or non verbal, like raising your finger up in the air in front of them. Once you choose a cue, use the same cue every time the horse smiles. After some practice, they should be able to perform their new trick as soon as they get the cue to smile.