I went to an interview and i was asked this on ASP.Net :

There is a button on client side and on click it calls the button click event on server side.

How does it interact with the event on server side while the button is residing in the client side , what exactly is the mechanism behind it?

I responded that the button click event is called as we write the event name on onclick, but the interviewer said how does it interacts with the server? 

I was not sure of the mechanism, can anybody explain?