easy!!!

Because lemon has citrus acid which is a colorless translucent crystalline acid, C6H8O7, principally derived by fermentation of carbohydrates or from lemon, lime, and pineapple juices and used in preparing citrates and in flavorings and metal polishes.
Citric acid is a relatively strong organic acid, and is very soluble in water. Citric acid and its salts are widely used because they are nontoxic, safe to handle, and easily biodegraded.

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Citric acid, because of its low toxicity, relative noncorrosiveness, and biodegradability, is also being used for applications normally reserved for the strong mineral acids.
Citric acid is the active ingredient in some bathroom and kitchen cleaning solutions. A solution with a 6% concentration of citric acid will remove hard water stains from glass without scrubbing.
I hope this can help with your understanding. =)