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Tayammum (Arabic: تيمم) is the Islamic act of dry Ritual purification using a purified sand or dust, which may be performed in place of ritual washing (wudu or ghusl) if no clean water is readily available or if one is suffering from moisture-induced skin inflammation or scaling.
Tayammum (Arabic: تيمم) is an Arabic word that means an aim or purpose. Tayammum drive from "amma" meaning to repair thing.
In Islamic Law, Tayammum meaning to wipe the face and hands of a person with the purpose of purification and providing oneself to pray and et by Aiming for or exploring soil, purified sand, or dust.
"O you who believe! When you intend to offer As-Salat (the prayer), wash your faces and your hands (forearms) up to the elbows, rub (by passing wet hands over) your heads, and (wash) your feet up to ankles. If you are in a state of Janaba (i.e. after a sexual discharge), purify yourselves (bathe your whole body). But if you are ill or on a journey, or any of you comes after answering the call of nature, or you have been in contact with women (i.e. sexual intercourse), and you find no water, then perform tayammum with clean earth and rub therewith your faces and hands. Allah does not want to place you in difficulty, but He wants to purify you, and to complete His Favour to you that you may be thankful."[a]
In the following seven circumstances, one has to make Tayammum rather Wudu (ablution) or ghusl (a ritual bath): 
O you who have believed, do not approach prayer while you are intoxicated until you know what you are saying or in a state of janabah, except those passing through [a place of prayer], until you have washed [your whole body]. And if you are ill or on a journey or one of you comes from the place of relieving himself or you have contacted women and find no water, then seek clean earth and wipe over your faces and your hands [with it]. Indeed, Allah is ever Pardoning and Forgiving."[b]
Thus, in case of illness, even if there is water, one still does not need to wash.
The same conditions that invalidate wudu also invalidate tayammum. In addition, a person's tayammum is invalidated as and when water becomes available.
Tayammum is permitted on clean earth piece, but the ideas about what is inside of this definition or not is depending per maddhabs. Some maddhabs accepts baked earthen pots (unglazed), clay, limestone, the tayammum stone, taahir (pure) earth, and walls of mud, stone or brick.
Tayammum is not permitted on things that are not earthen, such as things that burn and turn into ashes or things that can be melted by heat. Examples include: