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Counterfeit banknote detection pen
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<p>A counterfeit banknote detection pen is a pen used to apply an <a href="/facts/Iodine/JlfMecMf">iodine</a>-based ink to <a href="/facts/Banknote/mm6H2XI4">banknotes</a> in an attempt to determine their authenticity. The ink reacts with starch in wood-based paper to create a black or blue mark but the paper in a real bill contains no starch, so the pen mark remains unchanged. </p>