Salt

[natalie_1982]

How does salt dehydrate you if it retains water in yourbody and you need salt tablets to cross the desert and stuff?

[Jenny Tools]

Salt absorbs water and you don't need salt tablets to cross a desert (thats a fallacy) you need a camel. 

[brachiopod]

Ingesting (high doses of) salt causes fluid to be lost from the cells by osmosis through the cell membrane, having a dehydrating effect.

Salt is lost through sweating, and prolonged exposure to a hot, arid environment can have implications, in terms of salt depletion, for an un-acclimatised person.

Salt (sodium chloride) only absorbs water in its solid, crystalline form, or in saturated / super-saturated solution. (It is hygroscopic and slightly deliquescent, which means it absorbs water from the atmosphere and (under suitable conditions) will itself dissolve in the absorbed water.)