How can you stop your dog from chasing squirrels ?

the Leave it command is a great option! Is best to start small and simple when learning "leave it". Begin with treat in open palm directly in front of the dog, say leave it and if they make a move to eat it quickly close your hand saying "no leave it". once they make no move on the open palm, go ahead and say OK! for them to take the treat. Continue this training progressively getting harder, like putting the treat farther away. Then tossing it or using a favorite toy to mimic the movement of an animal.

Dogs naturally have a prey drive and will chase small things that move: balls, toys, other dogs, cats, squirrels, etc. You can divert their attention. You can also train your dog to "Leave It" and to come when called. There are training games that help your dog learn to be patient and not go after whatever it sees.

Do what you can to make your yard safe for your dog: fence or tether. The chasing may mellow out as he/she gets older, but it might not.

I honestly think that if your dog is young, it is just a curiosity that we cannot do anything about! The curiosity will reduce by the time the dog gets older. However, I do agree that if could be dangerous and uncontrollable often times. Maybe, draw your dog's attention to something else, whenever your dog witness a squirrels or cats. Or how about clicker training your dog?