From the smallest fly to the biggest elephant, and from fish living at the bottom of the ocean to birds that fly several kilometres above land: this is the animal kingdom, the biggest group of living things in the world. Some are very different, others are the same in many ways - but these mammals, birds, fish, reptiles, amphibians, and invertebrates have all managed to live for thousands and thousands of years. How do they find food, grow, keep safe, and have young - and what is the future for them in this fast-changing world?