Tayammum is the Islamic rite of "dry ablution," a purification performed with clean earth or dust in place of water when water is unavailable or its use would cause harm. The Qur'an itself provides for it: in the verse commanding ablution before prayer, it directs those who can find no water, travellers, the sick, those for whom water is out of reach, to turn instead to clean earth, wiping the face and the hands with it, so that the obligation of purity before prayer need never be an impossible burden. Tayammum thus substitutes for both the lesser ablution of wudu and the full washing of ghusl when water genuinely cannot be used.
The rite is simple: with the intention of purification, the worshipper strikes or touches clean earth, dust, sand or stone with the palms and wipes the face and then the hands and forearms. It is valid where water is absent, where the little water available is needed for drinking, where illness or injury would be worsened by washing, or where reaching water would bring real hardship or danger. Islamic law treats tayammum as a genuine dispensation and a mercy, and the tradition draws from it the wider principle that the religion is not meant to place unbearable difficulty upon its followers, and that God desires ease for them rather than hardship.