Animals are multicellular eukaryotes that can move, breathe, acquire food and digest it, have sensory and nervous systems, and respond quickly to stimuli. They have also developed specialized cells that are different from unicellular organisms such as bacteria, fungi and plants.

They are able to move freely at will because of muscles and other locomotory structures, which can use energy, such as ATP (adenosine triphosphate), to propel them from one place to another. This ability is essential to their survival and reproduction.

Many animals, including birds, mammals, reptiles and fish, have a life cycle that includes periods of growth and development as well as times of death. This allows children to learn about the full spectrum of life and helps them develop empathy for living things.

Most animals can detect and respond to various stimuli, such as light, sound, touch and pressure. This is important because it enables them to find and interact with their food, mates, shelter and other necessities.

They can also sense danger and escape from predators and other threats. This can help them avoid harm and protect themselves, their family and the environment.

Besides being a source of companionship, pets can provide kids with an opportunity to observe and participate in a variety of natural processes. This can be especially helpful for young children with learning disabilities, autism or other special needs.

Pets can also help reduce stress, anxiety and depression. Studies have shown that having a pet can decrease levels of cortisol and increase feelings of social support.

In addition, having a pet can give children a sense of security and help them to feel safe in their own homes. In fact, studies have shown that having a dog or cat can reduce burglary and theft rates in neighborhoods.

Most animals have a bilaterally symmetric body plan, meaning that the two sides of their bodies mirror each other. This is a feature that has helped them survive in their natural environments for millions of years.

These features make animals very useful for scientific research, particularly in areas such as genetics and biology. They are also used in the field of biotechnology, where they serve as model organisms for the development and testing of pharmaceutical drugs, chemicals, medical devices and other products.

They are also an invaluable resource for the prevention of diseases. Vaccines, for example, are often tested in monkeys before they can be used on human patients.

The animal kingdom is composed of more than 40 phyla, or groups of related organisms. The majority are invertebrates, but there are a number of vertebrates as well, such as birds and fish.

Some of the more familiar animals include reptiles, mammals and fish. Other phyla include insects, crustaceans, worms, and corals.

All phyla have cells that contain collagen, a triple helix of protein that holds the cell together and provides it with strength and flexibility. Collagen is also present in sponges, but is absent in unicellular eukaryotes and fungi.