The secret history of invisible ink

Lemon juice, orange juice, onion juice; saliva, urine, blood, vinegar, aspirin, and a laxative: How to mix your own invisible ink…
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Lemon juice, orange juice, onion juice; saliva, urine, blood, vinegar, aspirin, and a laxative: How to mix your own invisible ink… more»

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News source: Arts & Letters Daily

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