So there's this deaf American visiting Russia, and he's thirsty. Using American Sign Language, he says to his deaf-guide, "I really want a soda." But in Russian Sign Language, the gestures he used correspond to, "I really want to have sex." Guessing at some linguistic problem, the Russian guide diplomatically ignores the American's request and signs back with some small talk: "I'm from Moscow." Unfortunately, the RSL for "I'm from Moscow" corresponds to "I menstruate" in ASL.
The anecdote is probably apocryphal, but it illustrates an important point about sign languages. They are equivalent to spoken languages right down to the capacity for comic misunderstanding. And like spoken languages, the evidence is mounting that sign language develops through innate rather than culturally transmitted means. In other words, there is a language organ in the brain and we are all born with hard-wired rules of grammar.
The most compelling evidence for this comes from a school in Managua, Nicaragua.