Photo by Kevin H.

Have you ever wanted to make really good invisible ink? Well, the Central Intelligence Agency has a reliable recipe it’s now happy to share. As part of a large declassification of the U.S. government’s six oldest classified documents, the CIA has made publicly available a number of tips sure to amaze your six-year-old nephew, like how to make invisible ink and how to open envelopes without breaking the seal.

The documents date from the World War I era, according to the agency:

“These documents remained classified for nearly a century until recent advancements in technology made it possible to release them,” CIA Director Leon E. Panetta said. “When historical information is no longer sensitive, we take seriously our responsibility to share it with the American people.”

One document outlines the chemicals and techniques necessary for developing certain types of secret writing ink and a method for opening sealed letters without detection. Another memorandum dated June 14, 1918 – written in French – reveals the formula used for German secret ink.

What will you need to open up someone’s mail without being detected? Five drams copper acetol arsenate, three ounces acetone and a pint of amyl alcohol. Oh, and a bathtub to mix it all together in. Just…try to avoid inhaling the fumes.