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High-security couriers carry banknotes in boxes that release dye if snatched. But enterprising crooks have now found they can clean the defaced notes with a mix of weak acids and hydrogen peroxide. So security firm Brink’s has developed a watertight briefcase containing a small device that shatters a reservoir of concentrated sulphuric or nitric acid if the case is stolen (WO 03/004818). The acid sloshes round the container, eating away the cellulose in the banknote paper. All the crooks are left with is a fuming cash stew.