Stiles wasn't sure when it first occurred to him.
No.
That was a lie, he knew exactly when, it was a few days after the pool incident.
It was clear that Derek hadn't slept in all this time but it was when he yawned for the third time Stiles finally understood what cause his attention about it.
"Oh my god! You curl your tongue," Stile cried, "Just like a_" he cut himself off at the scowl and soft rumble coming from the man, his dark expression more than enough warning.
Well it should have been.
He tried to hold it in but quickly he dissolved into giggles.
Quick as a flash a fist shot out to whack him in the shoulder, but it did nothing, he collapse to the floor laughing hard.
"Can we get back on topic, now!" Derek snarled. Taking slow deep breath's Stiles slowly climbed to his feet.
"Okay, okay," Derek nodded, clearly relieved. Before his face crumpled into another yawn.
He didn't need to hear the thump and giggles to know that Stiles had collapsed with laughter again.
Vision clearing he growled and stalked off.