Bucky took it for as long as he could. But about halfway through the third hour, he sucked in an unsteady breath and said hoarsely, "Stop."

Instantly releasing his grip on Bucky's prosthetic arm, Tony said, "Do you need a break?"

Bucky took another breath. After a minute, he managed a nod.

"Sorry, yeah. Just give me a minute." He stood up and walked across the lab to a big bay window, trying to steady his breathing.

Watching him, Tony said, "You know, you don't owe me a damned thing. We don't have to do this today. Or at all."