Most people live in homes that are family fixed in one spot They may be huge apartment blocks or small private resdences, mud huts on stils, but they are alike in that they are not easily moved. For some people, however, who like to wander, a permanent dwelling is a handicap. They prefer a mobile one that they can pick up and take along with them on their travels Tents are convenienty portable, and for many centuries they have been a favorite form of shelter for nomadic trbesmen Tents are also frequently used in military operations, and on ordinary camping trips Other types of mobile homes such as caravans or houseboats, are roomier and more comfortable to live in than canvas tents o Certain animals also enjoy the advantages of living in a mobile home. The snail, which carries its house about on its back, manufactures for itself a durable shelter of exactly the right size. As the snail grows, it adds to its shell, so that it never needs to search ahout for larner quartprs A snail dnes not shed or exchange ts shel, it simply