By Elizabeth Cohen, CNN

(CNN) -- Every day for about five years, Israeli sisters Reut and Shoham Nistel ran home from school, made themselves sandwiches and plopped down on the couch to watch an Argentine telenovela with Hebrew subtitles.

The girls became so proficient in Spanish that they started speaking it at home to keep secrets from their parents.

"That's how we learned English, too," said Reut, now 26. "We had English class in school, but I never paid attention. All my English is from 'Full House' and 'Family Matters.' "

Although excessive screen time is often frowned upon, language experts say that watching shows in a foreign language -- if done with near obsession -- can help someone learn that language.

"These stories are hugely common," said Melissa Baese-Berke, associate professor of linguistics and director of the Second Language Acquisition and Teaching program at the University of Oregon.

Baseball players learned from 'Friends'

She points to a New York Times story about professional baseball players from Latin America who learned English by watching "Friends" with Spanish subtitles.

But they didn't just watch "Friends"; they watched it over and over again. Philadelphia Phillies shortstop Freddy Galvis told the Times that he had watched every episode of the 10-season show at least five times.

Stephen Snyder, dean of language schools at Middlebury College in Vermont, said this story sounds familiar to him.

"Our Japanese classes are full of Chinese students and American students who grew up watching Japanese anime, and without having any formal training in Japanese, their comprehension is quite reasonable," he said. "It's a transnational phenomenon, and it makes sense."

Baese-Berke says science supports what these young people have experienced. Studies show that it's best to acquire a language through both active and passive learning, and watching shows in a foreign language involves both.

Trying to figure out a word that a character in a telenovela is saying would be an example of active learning, and admiring the character's outfit while hearing Spanish in the background would be an example of passive learning, she said.

The 3 keys to learning from TV

Baese-Berke said there are three tricks to learning a foreign language through a show.

First, it has to be highly engaging. The Nistel sisters, for example, never missed an episode of "Chiquititas," the Argentine tween musical telenovela that was enormously popular among Israeli middle-schoolers in the late 1990s and early 2000s. The fact that their classmates talked about the show obsessively increased their devotion.

Second, it's best if the show has subtitles, so when viewers hear a new word, they can look down and find it in written form in their own language.

Third, the storyline should be repetitive. In "Chiquititas," for example, a group of plucky orphans are forever falling in and out of love and overcoming life's obstacles. "Friends" has similar storylines about 20-somethings in New York City.

"Telenovelas have a predictable structure: They have a problem, and they find a solution. You can follow the plot pretty easily," Baese-Berke said.

She and other experts add that although watching shows goes a long way, it's best to pair it with formal language training to learn grammar and structure.

Children might naturally learn languages more easily, but the telenovela technique can work with adults, too.

Vardit Ringvald, a professor of languages and linguistics at Middlebury and director of the school's Hebrew program, said she learned Spanish by watching "Andrea Celeste," another Argentine telenovela.

"When I married my husband, who's from Uruguay, I didn't speak a word of Spanish," she said. "After three months of watching 'Andrea Celeste,' I was fluent."

Soon, she and her husband were speaking Spanish to keep secrets from her mother.

"But we can't do that anymore, because my mother started to watch telenovelas, and now she's fluent, too," Ringvald said.