Atleast once a day I'm going to post a new fact, explain it, and give you a link to a more in depth explanation.
Most computer software refuses to do anything with images of money due to an agreement with various governments.
But how can the software tell?
If you look at your cash, You will see a series of patterned dots. These dots are in a specific pattern which the software can recognise, and then refuse to do anything with the image. This certainly applies for Photoshop, the most commonly used image software.
