Since it hit the airwaves in 2003, NCIS has become one of America's favorite shows, and we're still tuning in each week 15 seasons later.

If you keep an eye out, you might notice sneaky references and hidden jokes in certain episodes. NCIS super-fans have picked up on lots of clever bits of trivia that you probably missed. Here are 11 of our favorites:

1. Cote de Pablo needed a Hebrew tutor

While she plays the Israeli Mossad agent Ziva David on the show, de Pablo is actually a Chilean-America. Whenever Ziva spoke Hebrew, she had to depend on her language tutor to help get it right. But de Pablo has her own talents. She did all her own stunts, and also used her own voice whenever Ziva sang in an episode.

2. Most Wanted

Keep an eye on the Most Wanted board in the NCIS office. Throughout the different seasons of the show, the faces on it change. Actual bad guys from the show are included, but so is Osama Bin Laden - whose photo was crossed through with a big red line after he died.

But eagle-eyed fans will notice agent G. Callen from NCIS: LA and series creator Donald P. Bellisario on the board in certain episodes. Pretty sneaky.

3. The fade to black and white

Here's a tricky trivia question that will stump even the most devoted NCIS fans: what is the noise we hear when the show goes to commercial? Some say it sounds like a camera shutter clicking, but it's not.

The sound was actually made by Donald P. Bellissario hitting a microphone with his hand. He needed something recognizable and dramatic for the sound, and decided to take matters into his own hands.

4. Abby is based on Pauley Perrette

Producers for NCIS admit that fan-favorite character Abby was heavily inspired by Pauley Perrette, the actress who plays her. Perrette has a college degree in forensics and a masters in criminology. Like her character, she was born in New Orleans, and when we see a photo of Abby's adoptive mother on the show, it's just a picture of Perrette's actual mom.

The one big difference between them is that Perrette is actually blonde. She once had an allergic reaction to her black hair die, which sent her to the emergency room after her face swelled up to twice its size.

5. What exactly is a Caf-Pow?

Abby's energy drink of choice isn't real, sadly, but you can buy Caf-Pow merchandise. For the authentic NCIS experience, fill your mug with Hawaiian Punch. That's the drink that was used until Perrette cut sugar out of her diet, and the show switched to using unsweetened cranberry juice instead.

6. "Ducky" has had a long career

While David McCallum plays the NCIS team's mild-mannered medical examiner, he had a former life as a Russian spy. McCallum played Soviet secret agent Illya Kuryakin on the 1960s TV show The Man from U.N.C.L.E. They even reference his former role on the show.

Kate once asked Gibbs "What did Ducky look like when he was younger?" Gibbs responded "Illya Kuryakin."

7. All in the family

Sean Murray, who plays Timothy McGee on the show, is creator Donald P. Bellisario's step-son. In another family connection, McGee's sister on the show was played by Troian Ballisario, his real-life step-sister. Speaking of family members...

8. The young Agent Gibbs

Ever notice that when they show Agent Gibbs in a flashback, the actor who plays him has an uncanny resemblance to Mark Harmon? That's because Harmon's real-life son Sean plays the younger version of his character. Pretty clever.

9. DiNozzo's best joke

While fans are still mourning Michael Weatherly's departure from NCIS, he left behind a lot of laughs as Agent DiNozzo. But one of his jokes had a kernel of truth to it.

In a season three episode, DiNozzo quipped that he's "got a better chance of hooking up with Jessica Alba than these guys do infiltrating SeaLift." Weatherly and Alba were actually engaged between 2000 and 2002, after meeting on the set of Dark Angel. These days, Weatherly is happily married to his wife Bojana Jankovic.

10. The show was inspired by a vet's real experiences

Donald P. Bellisario was a Gunnery Sergeant in the U.S. Marine Corps in the 1950s, and he resented the way many veterans were depicted as crazy outcasts on TV. With JAG, which ran for 10 seasons, and its spin-off show NCIS, which is in its 15th season, Bellisario has helped create lots of positive depictions of veterans.

11. Abby's farting hippo has been the target of crimes

One of the show's most unlikely (and popular) recurring characters is Bert, Abby's farting stuffed hippo. While CBS sells licensed versions of the stuffed toy, some crew members have allegedly tried to take off with the real thing. Pauley Perrette says the gassy hippo had to be kept under lock and key in the props department to keep him safe.

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