Sonic loved the days Eggman was inventing.
Granted he never let on that he watched though, the Doctor liked his privacy when he was scheming or tinkering with his bots. But those moments were nothing compared to Eggman's face when he was creating a new invention.
The harsh lines from scowling smoothed away, making him look ten years younger. The tinted glasses he wore would be raised to sit atop his bald head revealing blood red eyes surrounded by black. Those eyes glowing as gloved hands cradled a vial here or measured a line there. Deft in their movements hands danced around the laboratory table knowing exactly what to take how much to take and when to take.
Seeing delight pass through those red and black eyes as he finished with an invention always made Sonic smile. As instead of an evil laugh a small happy chuckle would escape and pearl white teeth would flash for an instant.
Depending on the day, Eggman either played soft classical music or upbeat techno. He would tap his boot clad feet along with the music, a small sway of his hips as he really got into the project he was working on and the song he was listening too. Humming along and not once getting distracted from his task.
Though Sonic would love nothing more than to interrupt and really watch those hips move, he just could not bring himself to stop the true happiness on his lovers face. Of course it never stops him from requesting a show later on that night.