Cactus as a source of water??

what do you think about growing cactus as a source of water.....even if it has to be purified? I read somewhere that cactus can be a source of water, in emergencies? However, I am not a botanist and I honestly have no idea. How much of it would one need to grow to fill, say, a 1 liter bottle? Any ideas?

sorry...this thread got posted twice. Mods, please delete this one. Use the other one instead.