Eli Manning and Brandon Marshall were coming out of the shower on Tuesday after practice when the quarterback, out of nowhere, began flashing signs to the wide receiver.

And there was nothing strange about that at all.

At least not to Marshall, who is starting to get used to these impromptu learning sessions with Manning, periods of discussion that are unscheduled but instead occur on “Eli Manning Time.”

“It’s just real spontaneous,” Marshall said. “I mean, it’s interesting. We may be talking about what type of cereal we’re going to have for breakfast, and he just switches the subject to some type of football . . . Literally we just got out of the shower, we had towels on, and he was pretty much half-dressed, and he started giving me signals.”

That’s what it is taking, it seems, to get Marshall up to speed in the Giants’ offense. While Manning is entering his fourth year in the system along with Odell Beckham Jr., and others like Tavarres King and Sterling Shepard have last year’s experiences to draw from, Marshall is starting fresh. And, he said, despite bouncing around the league for a dozen years and having experience with a number of schemes, this one is completely different from anything he has ever had to digest.

“I feel really comfortable where I’m at right now,” Marshall said. “Obviously I’m not where I need to be, but where I was in OTAs and spring and summer wasn’t a great place as far as feeling like a rookie, new terminology . . . I’ve never really been in a no-huddle offense, the tempo is like learning a new language because everything is non-verbal communication. So, that was difficult for me, but we stuck to it and I’m in a really comfortable place now.”

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There are still, he admitted, areas that need work and refining.

“He’s probably the most detailed quarterback I’ve been around,” Marshall said of Manning. “He knows what he wants and how he wants it. It took me up until two weeks ago to get this one route down. He threw a ball into the ground and I was two steps off. Literally two steps off, and he pulled me to the side and just kept coaching me and walking me through it. Any other quarterback, it would have been perfect, right? But with this guy, no, it’s two steps. So, he’s special.”

Manning doesn’t only challenge Marshall in the shower, either. In practice on Tuesday Manning changed up the hand signals he uses because the Giants defensive backs were starting to pick up on them. The change didn’t register with Marshall, though, and Manning threw a pass to the sideline when Marshall cut inside.

“I thought he did it on purpose,” Marshall said. “I think he knew I didn’t have that one down. He’s like, ‘You’ve got to know it!’ So, I won’t make that mistake again . . . I’ve never seen that one [signal] before, I told him, but now I’ve got it.”

It may be a season-long process for Marshall.

“We’re not going to be where he and Odell are right now,” Marshall said. “They know each other so well, and it takes time. It could even take years . . . We’ll be fine. We’ll have some good chemistry this year, I can say that. But I can’t put any timetable on it.”

Well, there is one timetable. The season opens in Dallas on Sept. 10.

Tick, tock. That’s why Manning has been on Marshall.

“He’s challenging me every second he gets,” Marshall said, “so we’ll be where we need to be come Week One.”