Using the Phree pen, you can write anything anywhere. It really works: all the symbols written on any surface will be digitized and sent to a smartphone or computer. It’s an indispensable thing if you need to write something down urgently and have neither a phone nor paper at hand.
You can find it on Amazon.com! Look in the "Sports & Outdoors" tab for "Sawyer Products B4 Extractor Pump Kit". The price right now is $16.99.