Squirrels can be trained to be hand-fed. Because they are able to cache surplus food, they will take as much food as is available. Squirrels living in parks and campuses in cities have learned that humans are typically a ready source of food. Squirrels are occasionally kept as household pets, provided they are selected young enough and are hand raised in a proper fashion. They can be taught to do tricks, and are said to be as intelligent as dogs in their ability to learn behaviors. In these cases, a large cage and a balanced diet with good variety will keep a pet squirrel healthy and happy. As a pet, the owner must be aware of spring fever at which time a female pet squirrel will become very defensive of her cage, thinking of it as her nest, and will become somewhat aggressive to defend the area. Pet squirrels are usually kept without cages.