Ever since Windows 2000, the NTFS file system in Windows has supported Alternate Data Streams, which allow you to store data “behind” a filename with the use of a stream name. It’s not detectable while browsing the filesystem, or anywhere within Windows… you can only access it with the “secret key” which is really just the name of the stream.

You can think of these extra streams as secret compartments within the file, that can only be accessed if you know the “secret code”, which in this case is just the name of the stream.
This isn’t a completely secure way to hide data, but it’s a fun trick to know about in a pinch.
Note: This only works on a drive formatted with NTFS.

Hiding Data in a Secret Compartment

In order to use this feature, you’ll have to open a command prompt and use the following syntax:
notepad SomeFile.txt:SecretWordHere.txt

If you didn’t specify .txt on the end, Notepad will automatically add it, and ask if you want to create a new file, even if SomeFile.txt already existed, because SecretWordHere.txt doesn’t already exist.

Now you can enter in whatever data you want here and save the file:

When you look at the file, it will still be the exact same size as before