Because they are rodents, hamster incisors grow constantly. They need wooden blocks and hard food to wear the teeth down and prevent them from getting too long. Most hamsters also have a wheel to run on for exercise. A 6 inch (15 cm) wheel is recommended for most dwarf hamsters, and at least an eight-inch (20 cm) wheel for Syrians. If the wheel is too small, the hamster's spine can become bent. Some people get other kinds of exercise equipment for their hamsters, like a "hamster ball" in which the hamster can roll around the floor (though these are not recommended by some animal welfare groups), or a long network of tubes with air holes for hamsters to crawl through. They should live in a tank or cage. Hamsters are not recommended for young children.