You may feel this question is really basic or consequently really tough to explain.

as we know sin=Opposite/Hypotenuse

so if you have given some value of Opposite side and some value of Hypotenuse then you can easily calculate the sine.

But What I really wanted to know is How sine ends up to be one of the side in right angled triangle and Why only the particular side is sin and not other.

I maybe not explaining it well.If so please let me know , I can try once again to explain , otherwise.