Credit: Blue Star Families

Who's quieter: a Navy SEAL or a ninja? It's a question Walker Greentree, 6, just couldn't get out of his head after his mom told him to, "Be quiet like a Navy SEAL."

"Walker is very curious, deliberate, detail oriented and loves to take things apart," his mother Vivian Greentree said.

Walker and his buddies, who had been playing outside their house in Alexandria, Va., quieted down after his mom's rebuke, but one of Walker's pals chimed in that ninjas are in fact quieter than SEALS.

"First I thought ninjas were quieter and then I wanted to change my mind," Walker said. "Ninjas, they spy on people, and SEALs they save people."

The debate was on. Walter, whose father is serving his fifth deployment overseas, kept asking family friends in the military community for their thoughts, so his mom suggested that he write a letter to Admiral William McRaven, a former Navy SEAL and commander of U.S. Special Operations Command.

"Dear Admiral McRaven," he wrote in the letter which has gone viral online. "When I grow up I want to be a SEAL too but, can you tell me who is quieter - SEALs or Ninjas? Also, how long can you hold your breath for?"

After patiently waiting, a package came from one of the nation's top Navy SEALS, Admiral McRaven.

"I think ninjas are probably quieter than SEALs, but we are better swimmers, and also better with guns and blowing up things," McRaven responded. "I can hold my breath for a long time, but I try not to unless I really have to."

Walker's favorite part of the package was the challenge coin he received from the admiral. In the military community, challenge coins symbolize unit identity and brotherhood among the US Armed Forces and can also be given as a reward.

"It's my special coin, " Walker boasted.

The letter is currently hanging on the fridge while it's waiting to be framed, but Walker's challenge coin goes everywhere he goes.

"He won't let me frame the coin," Vivian Greentree said. "He's been making sure that he wears pants with pockets so that he can put the coin in the pockets."

He even brought the coin to school to show his classmates and shared the news with his dad overseas, who explained the tradition behind it.

"My dad told me too that … when I lay it out, people at the bar can buy you a Sprite," Walker said. "[My classmates] thought it was cool, but nobody got me a Sprite."

Vivian Greentree, who is the research and policy director for the Blue Star Families Organization for military families, said it's great that Walker, and his brother, MJ, can share experiences like these with their dad while he's serving overseas.

"Because it's a way of life, I have to make sure that every deployment makes them stronger people and do it in a fun way," she said. "It was very wonderful to see him engaging with his dad overseas, it's priceless…"

When asked what's the coolest thing about being a Navy SEAL, Walker told me, "This is going to be difficult, [but] it's to save people."