You return, naked, carrying the lube.
"Okay," Natasha says, a gleam in her eye, seeing your naked body. "This may be a bit hard on you," she continues, "but I promise you a reward, if you follow my instructions, exactly."
"I'll do exactly what you say," you answer, quickly.
"Well, then, get a bunch of lube in your left hand, smear it all over just the head of your cock, then start stroking your head, as fast as possible. You can stroke a little of your shaft too, but mostly focus on your head. Go on, stroke, FAST!"