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Well first of all, cactuses is also accepted as the plural form of cactus. Although cactuses is more common in speech and ordinary usage, cacti is favored in print sources and for scientific uses.

Quite a few people are under the assumption, that you can just cut open a cactus and find some water to drink. However that is a way too optimistic assumption. The moisture found within a cactus is very acidic. This is because succulent plants (plants that store water) need to use a different variant of photosynthesis to avoid evaporation of stored liquids. This method is called Crassulacean Acid Metabolism. The plant would then only open their stomates (plant pores) at night and store the collected carbon dioxide as an organic acid called malic acid. During this process they will also produce oxalic acid. Oxalic acids are toxic to humans as they produce calcium oxalates (creates kidney stones) that can clog up our kidneys.

Cactus fruit apparently tastes quite bad and the cactus juices will cause diarrhea and vomiting if ingested in excessive amounts or on an empty stomach. This would then dehydrate you even further and kill you faster.