Copying pencils. They look like a pencil, generally smell like a pencil but have a magic function which is widely unnecessary in the modern world. Write with it, place a damp sheet of tissue or tracing paper on top and press it and you'll get a wet copy of the original text. I told you, magic!
It's not quite that simple. Copying pencil lead is a combination of graphite, clay, aniline dyes and silver nitrate. They were unerasable until the advent of the vinyl eraser and appreciated for their permanence. The copying pencils of the 19th and early 20th century are quite different from the ones found today, as their predecessors have been found to be mildly toxic. Though modern versions may not be quite as effective, they still have a place with artists who are looking for a strong, permanent mark.
You might ask why someone would want to use something that's toxic, old, and irrelevant. Historically, copying pencils were associated with conductors and referred to as 'railroad pencils'. During WWI they were used by British and American troops for war related paperwork as copies could be made and because they were less fussy than a pen and ink. Sadly, they fell in popularity shortly after the war when the ballpoint pen came to be.
Try one--aside from all the history and science they're just really fun to play with, especially when you consider that once upon a time they were considered to be so very convenient.