A secret. Because if you share it, it's not your secret anymore.
You can’t keep a secret for long. Research reveals that no matter how trustworthy you think you are, you'll eventually spill the beans. Here, as believed by social scientists, are some of the reasons why (featured on Salon.com):
Having secrets makes us feel sad.
Sharing secrets makes you healthier.
They make us feel burdened, literally.
Our brains won’t let us keep secrets for too long.
For more studies found about keeping a secret, see The Atlantic's Why You Can't Keep a Secret.
Ever wondered how long a woman can keep a secret? Well the answer, it seems, is less than two days. Researchers found that they will typically spill the beans to someone else in 47 hours and 15 minutes. Read more at DailyMail.co.uk.