How about if she had a really big secret?

Tom thought it would be interesting if there was something about Sarah that he didn't already know, some big secret that she hid for so long and so well that nobody, even him, didn't even suspect it.

Taking the coin, Tom closed his hand around it and murmured: "I wish Sarahhad a secret that would make it impossible to maintain our current day to day life. The secret is directly related to her, being that she actually is not human, or if she is human, she is not an ordinary human. She will decide to tell me herself or something will happen that will make me find out everything."

The coin heating up was the sign that the wish had been fulfilled, now he just had to wait for it to manifest itself.