We don't know, they're a secret.
A secret
If a company is profitable they may want to keep their marketing a secret. If a company markets their company in unusual ways, they may want to keep it a secret.
all rich men are commonly misers so they want to keep this as a very huge secret.
What big seceret did Mr. and Mrs.Dursley want to keep
Well, if you liked a guy what would you want him to do for you - tell or keep it secret? What you would want in the same circumstances is probably what you should do.
No, she did not keep her methods a secret.
Women like to keep their weight a secret because they don't want people to make fun of them if they are a little overweight.
You don't tell anyone.
Bella wanted to keep the fact that her transforming into a vampire was happening sooner than Edward had anticipated a secret from him.
It wouldn't be a secret anymore if I told you, now would it. Some things, celebs want to keep to themselves. That's why it's a secret hobbie.
He keeps himself hidden because (if you read the secret series) he does not want to be caught by the midnight sun members. Therefore, he must keep his name a secret.
because she dosent want to make her fans cry :)