When Judy logged on the network, she faced the message requesting that she changes her password. So, she changed her password. But, she does not like to remember too many passwords. She wants to keep only one password. So, she tried to change the password again to her original password. But, she cannot change the password. Why can't she change the password

Because reusing the old passwords possess security threats.

Explanation:

A password can be defined as a string of characters or words or phrases that are used to authenticate the identity of the user. It is also known as passcode and should be kept confidential. A password is used to access constricted systems, applications, etc.

A password or passcode is usually composed of alphabets, numbers, symbols, characters, alphanumeric, or a combination of these.

In the given case, Judy was not able to change her passcode to the previous one because reusing old passwords is prohibited in any sites or systems. A system denies the user to reuse the old passwords for various reasons but most importantly due to security reasons. Though it is said that old passwords can be used after 100 times but seldom someone changes a password that much. All systems care for the security of their users thus they deny reusing old passcodes.