Bob, Jane and I keep the same secret. No one else in the world knows except for the three of us. One day, I told the secret to someone outside of us three. Still, no one else in the world knows except for Bob, Jane and I. What was the secret? (Hint:This is not a language or naming trick)
The secret is;
Three of us know it!
originally three people know the secret, when you tell it to another person, he/she includes in us(four people in us) but only three of us knows it(secret itself) and no one except us knows it(makes this statement true as well). so doesn't matter how many includes in us but both the statements will be true always.