Hamster balls are hollow spheres made of clear plastic

Plastic

A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

 into which hamsters, gerbils, degus and other small rodent

Rodent

Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....

 pets are placed, allowing them to run around outside their cages without the risk of running away, getting lost under furniture or in walls. The balls also produce an audible rumble across many surfaces, making the hamster easier to find.

Most hamster balls are made of durable, transparent plastic

Plastic

A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

 with air holes and a small door

Door

A door is a movable structure used to open and close off an entrance, typically consisting of a panel that swings on hinges or that slides or rotates inside of a space....

 or lid

Lid

LID is an abbreviation for:* Local light injection and detection, a method to determine fibre splice losses, especially in polarisation-maintaining fibre splicing...

to allow the owner to insert or remove the hamster from the ball. Hamster balls are supposed to provide a good workout for hamsters so they keep in good shape.

Although hamster balls are designed to protect hamsters, there are hazards such as stairs and other high places from which hamsters can fall, resulting in injury or even death. It is advised that any hamster owners with hamster balls must keep them on the floor on the lower levels of the house, away from any stairs. If you are planning to let the pet in the ball near the stairs, then you should create a simple barrier to stop them rolling down the stairs. Children should not throw, drop, or bounce hamster balls.

Hamster and other small pet owners should also be advised that it is important to keep watch over the hamster ball at all times while the pet is inside. Some hamster owners have noted that at times the ball can pick up too much speed, causing the hamster to somersault inside and potentially be injured.

Hamster balls are advised by the manufacturers for use only with hamsters, gerbils, mice, degus and small rats; they should not be used for rabbits, chinchillas or guinea pigs, as their body structures are different from small rodents, posing a serious hazard to them if placed inside.