Watching a dog chase his or her tail can be funny, but if it seems like Fido is doing it all the time, you need to take steps to stop the behavior before it becomes obsessive.
The goal is to stop your dog from chasing his or her tail, so you need to prepare yourself to correct this behavior quickly each and every time it happens. Correcting the behavior only some of the time will cause the dog to become confused ("Well, you let me do it 10 minutes ago."), so consistency is the name of the game. Likewise, if you correct too slowly, your dog will not learn. Timing is critical.
What you need:
A noise-making device: This can be as simple as clapping your hands, or you can make a noisemaker by taking an empty plastic bottle, such as for single serving juice or soda, and putting plenty of small rocks, marbles or bearings inside, then sealing the bottle. Shaking this makes the right noise to distract the dog from the negative behavior. A loud, "NO", can do the job, but if you have to do it frequently, it's probably better to use another noise-making method and save your voice.
A toy that the dog loves: When you stop an unwanted behavior, you need to replace it with a positive behavior, such as playing with a toy.
Have these tools ready at all times.
Observation:
Observe your dog closely. Look for things that come right before the dog chases its tail. Does he get excited? Is it only when she's playing with another dog?
Timing:
The sooner you catch the behavior when it starts, the faster your dog will learn.
When you see the very beginnings of tail chasing, make the noise - whether it's clapping your hands or shaking the bottle with rocks in it. The key is to make the noise loud, short and sharp. Your goal is to surprise and distract the dog from the behavior. Don't give Bowser a heart attack - just surprise him enough that he stops the behavior dead. Once she stops chasing her tail and looks around, quickly say, "Good dog!" and hand her the toy. By doing it this way, you are telling him that the tail-chasing is not an acceptable behavior, and you will give him a behavior he can do.
With keen observation, split-second timing, repetition and a bit of patience, you can stop your dog from chasing his or her tail.

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