Children have been writing secret messages using lemon juice for years, and even spies in World War I used such invisible inks to carry messages—sometimes unsuccessfully. But now scientists have created a more secure invisible ink for the modern age. Here’s how it works: First, you add a molecular “sensor”—called s-SMS—to another substance, anything from vinegar to mouthwash to household cleaners. The sensor molecule, which researchers described last week in a paper in Nature Communications, lights up depending on which substance you add. That glow can be measured at different wavelengths, which can be used to create a cipher and translate your message to code. After sending the coded message—along with just a little bit of s-SMS—to your co-conspirator, they dissolve the s-SMS into the same substance and reverse the process. This all might sound like a lot to handle, but 10 untrained users were able to decrypt messages using the technique. For an international spy with a handheld spectrometer, it could be a handy way to send messages while avoiding cyber-snoops. But for kids looking for a bit of after-school fun, lemon juice might still be the best option.