Why a violin can play more than one sound at the same time?

It is possible for a violin to play more than one note at the same time, and it's actaully a standard technique on playing violin named double stopping. It can be done because there are 4 strings for a standard violin. One can use the bow at a correct angle to attack two strings at the same time and play two notes at the same time.

Some more advanced techniques called triple stopping or quardruple stopping are done similarly on attacking 3 or 4 strings at the same time (or merely at the same time) to play 3 to 4 notes (merely) at the same time.

In my memory, I've seen and heard a violinist playing a triple stopping beautifully and it's really virtuoso, but I haven't come across a quadruple stopping yet as in my opinion it is a really rare and complicated technique since you have to alter the angle of attack smoothly through you're bowing.

But it would be quite common when the violinist do plucking.

Note that string instruments(like violin)can somtimes play two notes simultaneously(or even three or four).Wind instruments,however can only one at a time