* Lay eggs. This is the most striking difference between monotremes and other mammals.
* Have a cloaca. This is a single opening for the digestive, urinary, and reproductive systems.
* Possess a pouch (in some species). This is where they raise their young, similar to marsupials.
* Have a low metabolic rate. This means they don't generate as much internal heat as other mammals.
* Have electroreceptors in their snouts. This helps them detect prey in the water or underground.
These characteristics make monotremes a unique and fascinating group of mammals.