Squirrels are often the cause of power outages. They can readily climb a power pole and crawl or run along a power cable. The animals will climb onto power transformers or capacitors looking for food, or a place to cache acorns. If they touch a high voltage conductor and a grounded portion of the enclosure at the same time, they are electrocuted, and often cause a short circuit that shuts down equipment. Squirrels have brought down the high-tech NASDAQ stock market twice and were responsible for a spate of power outages at the University of Alabama. To sharpen their teeth, squirrels will often chew on tree branches or even the occasional live power line. Rubber or plastic plates, or freely rotating sleeves ("squirrel guards") are sometimes used to discourage access to these facilities.