Stiles' heartbeat was a frantic creature. Angered and desperate, it roared beneath the confines of flesh, intent on prying apart the bars that held it captive.
Its stuttering rhythm easily summoned Derek's attention, and for a moment he could envision broken ribs and viscera. A monster freed. A heart.
There's a reason it's called a cage.
"Your heart." The syllables were spoken in a taciturn grunt.
Stiles hummed an acknowledgment, the tone of the noise bordering an inquiry. Derek recognized it as an encouragement to continue speaking.
"It's fast," He managed.
Stiles glanced up at the comment, setting aside the worn tome in his grasp. His confused stare flickered across Derek's features, before realization blossomed in his gaze. His lips quirked upwards in a grin like white-noise; distant and constant.
"Worried, big guy?" There's a teasing lilt to his words despite the self-deprecating snort. "Don't sweat it. It's always been pushing one-fifty miles an hour on a sixty mile street."
The confusion must have shown on Derek's face because Stiles continued to elaborate, "Tachycardia. It's a heart condition. Means my heart sometimes throws parties and turns up the bass on all his favourite rock songs." His grin widened, voice adopting a conspiratorial tone, "The neighbours have filed noise complaints."
A beat passed. "I hope he got arrested." Derek deadpanned for lack of anything better to say.
Stiles released a sound of mock offense, "Of course not. He's has an in with the sheriff."
An exaggerated wink was shot Derek's way, and Stiles was sent spiralling into laughter at the patented Hale eye-roll he received in response. Derek stared on, helplessly, at amber eyes clenched in crescent moons and broad shoulders quaking with mirth.
His own beast awakened. It settled in his throat and pulsed there, racing as he tried not to notice that Stiles' laughter sounded like sunshine.
Derek sighed and it sounded like a confession.
A death sentence.
Or something equally as monstrous.