It started with a simple observation. "You know..." You rested your chin on your hand, watching Senku fill another notebook with calculations.

"I've never seen you get flustered."

"Hm."

"You've faced Tsukasa."

"Yep."

"Built a steam engine."

"Also yep."

You narrowed your eyes. "...But I've never seen you embarrassed."

Senku didn't even glance up. "Probably because embarrassment isn't useful."

"So that's a challenge?"

"No."

"I'm taking it as one."

"Tch."

"Do whatever keeps you entertained."

The next morning, Senku was bent over a cluttered workbench, carefully assembling a small glass device. You wandered over, hands behind your back.

"What're you making?"

"Vacuum filtration."

As he explained, you leaned over his shoulder to get a better look.

"Hm..."

"So this tube connects to—" He stopped. "..."

You looked down at him. "Senku?"

"...The tube."

He blinked once. "...Right."

"It connects to..." Another pause. You smiled to yourself.

"Did you just lose your train of thought?"

"No."

"You absolutely did."

"I was deciding the most efficient explanation."

"Mm-hm."

He continued talking, but he subtly shifted half a step away, pretending it was to reach another beaker. You bit back a grin.

Later that afternoon, you found him outside, notebook balanced on one knee as he calculated fuel ratios.

Sunlight filtered through the trees, catching the pale green tips of his hair. You sat beside him.

"You know..."

"Hm?"

"You look good when you're focused."

His pencil stopped. Only for a heartbeat.

"...Irrelevant observation."

You watched carefully.

"...Your ears are turning red."

"They're not."

"They definitely are."

"It's warm."

You looked up at the cloudy sky. "...It's winter."

"...Temperature is relative."

You laughed. "You've got an excuse for everything."

"Usually."

—•—•—•—

Three days passed.

You complimented him. You stood a little too close whenever he explained an experiment.

You casually fixed the collar of his coat once. Every time, Senku remained almost impossibly composed.

Almost.

Each time, there was the slightest hesitation. A tiny pause before he answered. A brief adjustment of his glasses. A faint flush at the tips of his ears that disappeared almost as quickly as it appeared.

Small enough that almost nobody would notice.

Almost nobody.

One evening, the laboratory was quiet except for the crackling fire. Senku was polishing a glass lens beneath the lantern's glow when you stepped inside.

"You've been working all day."

"Probably."

You walked behind him, reading the notes spread across the table.

Without thinking, you leaned over his shoulder again.

"So this formula..." Senku looked at the page. "...If you increase the concentration by..."

Silence. "..."

"...Senku?"

He stared at the equation. "...Right."

"The concentration."

You couldn't stop smiling. "You forgot what you were saying again."

"I didn't forget."

"It sounded like forgetting."

"...Temporary interruption."

"By what?"

He adjusted his glasses. "...An unexpected variable."

You tilted your head innocently. "Oh?"

He ignored the question and reached for another notebook. As he turned, a stubborn strand of green-and-white hair fell over his eyes.

"There."

Before he could react, you gently brushed it back into place. Your fingers lingered for only a second.

Senku froze. The notebook remained suspended in his hand.

"..."

"...Senku?"

He blinked once before clearing his throat.

"...You done?"

You stared.

"...Wait."

"You hesitated."

"No."

"You've never hesitated before."

"...Coincidence."

You stepped in front of him, smiling triumphantly.