A friend proudly boasts to you that his password is the day of the year that he was born, but converted to binary. You know he was born on the (135th) day of the year. What is his password?

Your social number is how the government can determine who you are (it’s your identity) and if someone has your social, then they have access to your identity. That means they can go committing crimes with your identity and have access to all your accounts and scamming with your identity and such!
So it’s important to keep it to yourself and the only time you would usually give it out is when your applying for a job.