A ship is a large buoyant watercraft. Ships are generally distinguished from boats based on size, shape and cargo or passenger capacity. Ships are used on lakes, seas, rivers, and oceans for a variety of activities, such as the transport of people or goods, fishing, entertainment, public safety, and warfare. Historically, a "ship" was a sailing vessel with at least three square-rigged masts and a full bowsprit. In armed conflict and in daily life, ships have become an integral part of modern commercial and military systems. Fishing boats are used by millions of fishermen throughout the world. Military forces operate vessels for naval warfare and to transport and support forces ashore. Commercial vessels, nearly 35,000 in number, carried 7.4 billion tons of cargo in 2007. As of 2011, there are about 104,304 ships with IMO numbers in the world. Ships were always a key in history's great explorations and scientific and technological development. Navigators such as Zheng He spread such inventions as the compass and gunpowder. Ships have been used for such purposes as colonization and the slave trade, and have served scientific, cultural, and humanitarian needs. After the 16th century, new crops that had come from and to the Americas via the European seafarers significantly contributed to the world population growth. Ship transport has shaped the world's economy into today's energy-intensive pattern.