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Spit and a handkerchief
Using spit would be very stupid. That is very bad advice.
Spit (saliva) contains electrolytes (ions) such as sodium, potassium, chloride, bicarbonate, and phosphate. It also contains mucus, enzymes, human cells, and bacteria. None of these are good for lens coatings or for optical clarity or for prevention of fungal growth.
Proper lens cleaning solutions (including condensed breath which is distilled water) do not have those chemicals in any significant concentration.