The flightless emu is the second largest bird in the world. Standing as tall as two metres, it can look an adult in the eye. It has a long neck, extremely strong legs and thick, drooping, double-shafted feathers. It cannot fly because it has no keel on its breast-bone to anchor flight muscles and no barbules (tiny hooks) to hold the feathers together. Emus are found only in Australia - both in woodlands and on semi-dry plains. They are mainly vegetarian - eating flowers, fruits and seeds - but they occasionally eat insects. They also swallow stones which help them to digest their food. An emu's legs are as powerful as a football player's. They need to support a big body (which weighs as much as 45 kilograms) while running at speeds of up to 48 kilometres per hour. Emus cover great distances in search of food and water but are nevertheless able to resist drought and, in extreme conditions, can go for several days without water. Tame emus are game for anything and have been known to pull the tails of sleeping dogs and then run away as fast as they can! In the wild, emus have been observed playing what seemed to be a game of tag. Nowadays, only one species of emu survives. The smaller species - once found in Tasmania and on small islands off Australia - were exterminated last century. Many say curiosity is killing the emu. Any strange sight or sound, or indeed anything unusual at all, is investigated by this bird. Hunters who knew of this natural curiosity once used it to catch the emu. The world's biggest bird is the ostrich.