When Linux switches from a normal user to an administrator, why do I prompt for a password with the SU command, and what key does the cursor not respond to?

Linux has strict permissions control, if you switch from root to other users do not need a password to switch, if you switch from another user to the root user must enter the root user's password.
1, Su Root prompted to enter the password because of high administrator privileges, in order to prevent other users from tampering with the system, switch to administrator must enter the administrator password.
2, the input password is not visible, the wrong can only re-lose, invisible cursor.

3. LINUX The password is ciphertext input. There is no display on the screen, not even ******.

When Linux switches from a normal user to an administrator, why do I prompt for a password with the SU command, and what key does the cursor not respond to?