I assume that the only secret is the key.

"It's certainly the only secret I can come up with to explain how we did it."

At first the only secrets were the mysterious letters used on the badge.

But this is not the only secret in the school.

But these are not the only secrets in the boarding school.

My only secret is to hang them high where hands can not get to them.

What you ask will take my last cent, and the only valuable military secret I hold.

It's the only secret of my race which we've kept from you till now.

Maybe because the only secret you'd kill for is what happened to you in Memphis.

"The only secret was the reason why he came over here," he said.