Metal
Lately, when he thinks he's going to be working on something into the night, he keeps his work area locked.
And, as she makes sure to remind him with great irritation the morning after she finds out, he hasn't given her the new code yet.
He tries to evade the issue, because he can't exactly tell her why she's suddenly being shut out of somewhere when she's used to unrestricted access.
One too many times, she's slipped in to offer a polite suggestion that three in the morning is late enough, and he's let her stay. And despite his vehement assertions that he wouldn't let some woman distract him, he's always found himself shoving her back against the nearest solid object, and all that cold metal digging into uncomfortable places has to spoil the mood.
And the last thing a guy wants is anything that gets his woman out of the mood.